diff --git a/src/Analyzers/CSharp/CodeFixes/Nullable/CSharpDeclareAsNullableCodeFixProvider.cs b/src/Analyzers/CSharp/CodeFixes/Nullable/CSharpDeclareAsNullableCodeFixProvider.cs
index 29af1616f6c1b..6882dc66da3d5 100644
--- a/src/Analyzers/CSharp/CodeFixes/Nullable/CSharpDeclareAsNullableCodeFixProvider.cs
+++ b/src/Analyzers/CSharp/CodeFixes/Nullable/CSharpDeclareAsNullableCodeFixProvider.cs
@@ -234,6 +234,7 @@ SyntaxKind.IndexerDeclaration or
             var symbol = model.GetSymbolInfo(invocation.Expression, cancellationToken).Symbol;
             if (symbol is not IMethodSymbol method || method.PartialImplementationPart is not null)
             {
+                // https://github.com/dotnet/roslyn/issues/73772: should we also bail out on a partial property?
                 // We don't handle partial methods yet
                 return null;
             }
diff --git a/src/Compilers/CSharp/Portable/Errors/ErrorCode.cs b/src/Compilers/CSharp/Portable/Errors/ErrorCode.cs
index c662639e55e9d..b4ba341c47edb 100644
--- a/src/Compilers/CSharp/Portable/Errors/ErrorCode.cs
+++ b/src/Compilers/CSharp/Portable/Errors/ErrorCode.cs
@@ -2310,18 +2310,6 @@ internal enum ErrorCode
         ERR_InterceptsLocationDataInvalidPosition = 9235,
         INF_TooManyBoundLambdas = 9236,
 
-        // PROTOTYPE(partial-properties): pack
-        ERR_PartialPropertyMissingImplementation = 9300,
-        ERR_PartialPropertyMissingDefinition = 9301,
-        ERR_PartialPropertyDuplicateDefinition = 9302,
-        ERR_PartialPropertyDuplicateImplementation = 9303,
-        ERR_PartialPropertyMissingAccessor = 9304,
-        ERR_PartialPropertyUnexpectedAccessor = 9305,
-        ERR_PartialPropertyInitMismatch = 9306,
-        ERR_PartialPropertyTypeDifference = 9307,
-        WRN_PartialPropertySignatureDifference = 9308,
-        ERR_PartialPropertyRequiredDifference = 9309,
-
         #endregion
 
         WRN_BadYieldInLock = 9237,
@@ -2337,6 +2325,17 @@ internal enum ErrorCode
         ERR_BadNonVirtualInterfaceMemberAccessOnAllowsRefLike = 9246,
         ERR_BadAllowByRefLikeEnumerator = 9247,
 
+        ERR_PartialPropertyMissingImplementation = 9248,
+        ERR_PartialPropertyMissingDefinition = 9249,
+        ERR_PartialPropertyDuplicateDefinition = 9250,
+        ERR_PartialPropertyDuplicateImplementation = 9251,
+        ERR_PartialPropertyMissingAccessor = 9252,
+        ERR_PartialPropertyUnexpectedAccessor = 9253,
+        ERR_PartialPropertyInitMismatch = 9254,
+        ERR_PartialPropertyTypeDifference = 9255,
+        WRN_PartialPropertySignatureDifference = 9256,
+        ERR_PartialPropertyRequiredDifference = 9257,
+
         // Note: you will need to do the following after adding errors:
         //  1) Update ErrorFacts.IsBuildOnlyDiagnostic (src/Compilers/CSharp/Portable/Errors/ErrorFacts.cs)
 
diff --git a/src/Compilers/CSharp/Portable/Errors/MessageID.cs b/src/Compilers/CSharp/Portable/Errors/MessageID.cs
index 449d25f533388..9c5288962c68b 100644
--- a/src/Compilers/CSharp/Portable/Errors/MessageID.cs
+++ b/src/Compilers/CSharp/Portable/Errors/MessageID.cs
@@ -287,8 +287,7 @@ internal enum MessageID
 
         IDS_FeatureRefStructInterfaces = MessageBase + 12844,
 
-        // PROTOTYPE(partial-properties): pack
-        IDS_FeaturePartialProperties = MessageBase + 13000,
+        IDS_FeaturePartialProperties = MessageBase + 12845,
     }
 
     // Message IDs may refer to strings that need to be localized.
diff --git a/src/Compilers/CSharp/Portable/FlowAnalysis/NullableWalker.Variables.cs b/src/Compilers/CSharp/Portable/FlowAnalysis/NullableWalker.Variables.cs
index 4eb4abe18b34c..dd3ed1448840d 100644
--- a/src/Compilers/CSharp/Portable/FlowAnalysis/NullableWalker.Variables.cs
+++ b/src/Compilers/CSharp/Portable/FlowAnalysis/NullableWalker.Variables.cs
@@ -396,6 +396,7 @@ internal Variables GetRootScope()
 
             internal Variables? GetVariablesForMethodScope(MethodSymbol method)
             {
+                // https://github.com/dotnet/roslyn/issues/73772: is this needed if we also delete the weird cascading in EnterParameters?
                 method = method.PartialImplementationPart ?? method;
                 var variables = this;
                 while (true)
diff --git a/src/Compilers/CSharp/Portable/FlowAnalysis/NullableWalker.cs b/src/Compilers/CSharp/Portable/FlowAnalysis/NullableWalker.cs
index 81109ab290733..faf378f8cfdaa 100644
--- a/src/Compilers/CSharp/Portable/FlowAnalysis/NullableWalker.cs
+++ b/src/Compilers/CSharp/Portable/FlowAnalysis/NullableWalker.cs
@@ -2764,6 +2764,7 @@ private void EnterParameters()
             }
 
             // The partial definition part may include optional parameters whose default values we want to simulate assigning at the beginning of the method
+            // https://github.com/dotnet/roslyn/issues/73772: is this actually used/meaningful?
             methodSymbol = methodSymbol.PartialDefinitionPart ?? methodSymbol;
 
             var methodParameters = methodSymbol.Parameters;
diff --git a/src/Compilers/CSharp/Portable/Generated/ErrorFacts.Generated.cs b/src/Compilers/CSharp/Portable/Generated/ErrorFacts.Generated.cs
index 9475412e96ddf..6e81f32c0f79e 100644
--- a/src/Compilers/CSharp/Portable/Generated/ErrorFacts.Generated.cs
+++ b/src/Compilers/CSharp/Portable/Generated/ErrorFacts.Generated.cs
@@ -341,8 +341,8 @@ public static bool IsWarning(ErrorCode code)
                 case ErrorCode.WRN_DynamicDispatchToParamsCollectionMethod:
                 case ErrorCode.WRN_DynamicDispatchToParamsCollectionIndexer:
                 case ErrorCode.WRN_DynamicDispatchToParamsCollectionConstructor:
-                case ErrorCode.WRN_PartialPropertySignatureDifference:
                 case ErrorCode.WRN_BadYieldInLock:
+                case ErrorCode.WRN_PartialPropertySignatureDifference:
                     return true;
                 default:
                     return false;
diff --git a/src/Compilers/CSharp/Portable/Symbols/PublicModel/MethodSymbol.cs b/src/Compilers/CSharp/Portable/Symbols/PublicModel/MethodSymbol.cs
index 054c99f464c2b..c3c9b8c4b1d61 100644
--- a/src/Compilers/CSharp/Portable/Symbols/PublicModel/MethodSymbol.cs
+++ b/src/Compilers/CSharp/Portable/Symbols/PublicModel/MethodSymbol.cs
@@ -298,8 +298,6 @@ IMethodSymbol IMethodSymbol.PartialDefinitionPart
             }
         }
 
-        // PROTOTYPE(partial-properties): Perhaps this API should be implemented as '_underlying.OriginalDefinition.IsPartialDefinition()' instead.
-        // However, this would be a behavior change. Callers may have been assuming that if the API returned true, then the receiver is an original definition symbol.
         bool IMethodSymbol.IsPartialDefinition => _underlying.IsDefinition && _underlying.IsPartialDefinition();
 
         INamedTypeSymbol IMethodSymbol.AssociatedAnonymousDelegate
diff --git a/src/Compilers/CSharp/Portable/Symbols/Source/SourcePropertySymbol.cs b/src/Compilers/CSharp/Portable/Symbols/Source/SourcePropertySymbol.cs
index 5f79f0dae2f0a..2deeefb254519 100644
--- a/src/Compilers/CSharp/Portable/Symbols/Source/SourcePropertySymbol.cs
+++ b/src/Compilers/CSharp/Portable/Symbols/Source/SourcePropertySymbol.cs
@@ -14,7 +14,6 @@ namespace Microsoft.CodeAnalysis.CSharp.Symbols
 {
     internal sealed class SourcePropertySymbol : SourcePropertySymbolBase
     {
-        // PROTOTYPE(partial-properties): Verify that the increase in memory consumption from this is acceptable
         private SourcePropertySymbol? _otherPartOfPartial;
 
         internal static SourcePropertySymbol Create(SourceMemberContainerTypeSymbol containingType, Binder bodyBinder, PropertyDeclarationSyntax syntax, BindingDiagnosticBag diagnostics)
@@ -175,7 +174,10 @@ public override OneOrMany<SyntaxList<AttributeListSyntax>> GetAttributeDeclarati
         {
             // Attributes on partial properties are owned by the definition part.
             // If this symbol has a non-null PartialDefinitionPart, we should have accessed this method through that definition symbol instead
-            Debug.Assert(PartialDefinitionPart is null);
+            Debug.Assert(PartialDefinitionPart is null
+                // We might still get here when asking for the attributes on a backing field.
+                // This is an error scenario (requires using a property initializer and field-targeted attributes on partial property implementation part).
+                || this.BackingField is not null);
 
             if (PartialImplementationPart is { } implementationPart)
             {
@@ -610,6 +612,7 @@ internal override void AfterAddingTypeMembersChecks(ConversionsBase conversions,
             if (IsPartialDefinition && OtherPartOfPartial is { } implementation)
             {
                 PartialPropertyChecks(implementation, diagnostics);
+                implementation.CheckInitializerIfNeeded(diagnostics);
             }
         }
 
diff --git a/src/Compilers/CSharp/Portable/Symbols/Source/SourcePropertySymbolBase.cs b/src/Compilers/CSharp/Portable/Symbols/Source/SourcePropertySymbolBase.cs
index 39f81325f36f6..d4fbc4a85099d 100644
--- a/src/Compilers/CSharp/Portable/Symbols/Source/SourcePropertySymbolBase.cs
+++ b/src/Compilers/CSharp/Portable/Symbols/Source/SourcePropertySymbolBase.cs
@@ -95,7 +95,7 @@ protected SourcePropertySymbolBase(
         {
             Debug.Assert(!isExpressionBodied || !isAutoProperty);
             Debug.Assert(!isExpressionBodied || !hasInitializer);
-            Debug.Assert(!isExpressionBodied || accessorsHaveImplementation); // PROTOTYPE(partial-properties): further adjust asserts?
+            Debug.Assert(!isExpressionBodied || accessorsHaveImplementation);
             Debug.Assert((modifiers & DeclarationModifiers.Required) == 0 || this is SourcePropertySymbol);
 
             _syntaxRef = syntax.GetReference();
@@ -279,20 +279,20 @@ explicitlyImplementedProperty is null ?
         internal bool IsExpressionBodied
             => (_propertyFlags & Flags.IsExpressionBodied) != 0;
 
-        private void CheckInitializer(
-            bool isAutoProperty,
-            bool isInterface,
-            bool isStatic,
-            Location location,
-            BindingDiagnosticBag diagnostics)
+        protected void CheckInitializerIfNeeded(BindingDiagnosticBag diagnostics)
         {
-            if (isInterface && !isStatic)
+            if ((_propertyFlags & Flags.HasInitializer) == 0)
             {
-                diagnostics.Add(ErrorCode.ERR_InstancePropertyInitializerInInterface, location);
+                return;
             }
-            else if (!isAutoProperty)
+
+            if (ContainingType.IsInterface && !IsStatic)
             {
-                diagnostics.Add(ErrorCode.ERR_InitializerOnNonAutoProperty, location);
+                diagnostics.Add(ErrorCode.ERR_InstancePropertyInitializerInInterface, Location);
+            }
+            else if (!IsAutoProperty)
+            {
+                diagnostics.Add(ErrorCode.ERR_InitializerOnNonAutoProperty, Location);
             }
         }
 
@@ -692,11 +692,7 @@ internal override void AfterAddingTypeMembersChecks(ConversionsBase conversions,
             this.CheckAccessibility(Location, diagnostics, isExplicitInterfaceImplementation);
             this.CheckModifiers(isExplicitInterfaceImplementation, Location, IsIndexer, diagnostics);
 
-            bool hasInitializer = (_propertyFlags & Flags.HasInitializer) != 0;
-            if (hasInitializer)
-            {
-                CheckInitializer(IsAutoProperty, ContainingType.IsInterface, IsStatic, Location, diagnostics);
-            }
+            CheckInitializerIfNeeded(diagnostics);
 
             if (RefKind != RefKind.None && IsRequired)
             {
@@ -1109,20 +1105,23 @@ private CustomAttributesBag<CSharpAttributeData> GetAttributesBag()
             // prevent infinite recursion:
             Debug.Assert(!ReferenceEquals(copyFrom, this));
 
+            // The property is responsible for completion of the backing field
+            // NB: when the **field keyword feature** is implemented, it's possible that synthesized field symbols will also be merged or shared between partial property parts.
+            // If we do that then this check should possibly be moved, and asserts adjusted accordingly.
+            _ = BackingField?.GetAttributes();
+
             bool bagCreatedOnThisThread;
             if (copyFrom is not null)
             {
                 // When partial properties get the ability to have a backing field,
                 // the implementer will have to decide how the BackingField symbol works in 'copyFrom' scenarios.
-                Debug.Assert(BackingField is null);
+                Debug.Assert(!IsAutoProperty);
 
                 var attributesBag = copyFrom.GetAttributesBag();
                 bagCreatedOnThisThread = Interlocked.CompareExchange(ref _lazyCustomAttributesBag, attributesBag, null) == null;
             }
             else
             {
-                // The property is responsible for completion of the backing field
-                _ = BackingField?.GetAttributes();
                 bagCreatedOnThisThread = LoadAndValidateAttributes(GetAttributeDeclarations(), ref _lazyCustomAttributesBag);
             }
 
diff --git a/src/Compilers/CSharp/Test/Emit2/Attributes/AttributeTests_WellKnownAttributes.cs b/src/Compilers/CSharp/Test/Emit2/Attributes/AttributeTests_WellKnownAttributes.cs
index ca2be38186095..5ce634fdea9c3 100644
--- a/src/Compilers/CSharp/Test/Emit2/Attributes/AttributeTests_WellKnownAttributes.cs
+++ b/src/Compilers/CSharp/Test/Emit2/Attributes/AttributeTests_WellKnownAttributes.cs
@@ -11220,6 +11220,45 @@ void local1()
             Assert.True(verifier.HasLocalsInit("C.<get_PropNoAttribute>g__local1|3_0"));
         }
 
+        [Theory]
+        [InlineData("[SkipLocalsInit]", "")]
+        [InlineData("", "[SkipLocalsInit]")]
+        public void SkipLocalsInit_PartialPropertyAccessor_ContainsLocalFunction(string defAttrs, string implAttrs)
+        {
+            // SkipLocalsInit applied to either part affects the property and nested functions
+            var source = $$"""
+using System.Runtime.CompilerServices;
+
+public partial class C
+{
+    {{defAttrs}}
+    partial int PropWithAttribute { get; }
+
+    {{implAttrs}}
+    partial int PropWithAttribute
+    {
+        get
+        {
+            int w = 1;
+            w = w + w + w + w;
+
+            void local1()
+            {
+                int x = 1;
+                x = x + x + x + x;
+            }
+
+            return 0;
+        }
+    }
+}
+""";
+
+            var verifier = CompileAndVerifyWithSkipLocalsInit(source);
+            Assert.False(verifier.HasLocalsInit("C.PropWithAttribute.get"));
+            Assert.False(verifier.HasLocalsInit("C.<get_PropWithAttribute>g__local1|1_0"));
+        }
+
         [Fact]
         public void SkipLocalsInit_EventAccessor_ContainsLocalFunction()
         {
diff --git a/src/Compilers/CSharp/Test/Symbol/DocumentationComments/DocumentationCommentCompilerTests.cs b/src/Compilers/CSharp/Test/Symbol/DocumentationComments/DocumentationCommentCompilerTests.cs
index 60a90b842ad31..a795619ee010d 100644
--- a/src/Compilers/CSharp/Test/Symbol/DocumentationComments/DocumentationCommentCompilerTests.cs
+++ b/src/Compilers/CSharp/Test/Symbol/DocumentationComments/DocumentationCommentCompilerTests.cs
@@ -1972,7 +1972,7 @@ void verify(CSharpTestSource source)
                     Assert.Equal("p2", indexer.Parameters.Single().Name);
                 });
                 verifier.VerifyDiagnostics(
-                    // (5,24): warning CS9308: Partial property declarations 'int C.this[int p2]' and 'int C.this[int p1]' have signature differences.
+                    // (5,24): warning CS9256: Partial property declarations 'int C.this[int p2]' and 'int C.this[int p1]' have signature differences.
                     //     public partial int this[int p1] => 42;
                     Diagnostic(ErrorCode.WRN_PartialPropertySignatureDifference, "this").WithArguments("int C.this[int p2]", "int C.this[int p1]").WithLocation(5, 24));
 
@@ -2033,7 +2033,7 @@ void verify(CSharpTestSource source)
                     // (4,42): warning CS1734: XML comment on 'C.this[int]' has a paramref tag for 'p2', but there is no parameter by that name
                     //     /** <summary>Accepts <paramref name="p2"/>.</summary> */
                     Diagnostic(ErrorCode.WRN_UnmatchedParamRefTag, "p2").WithArguments("p2", "C.this[int]").WithLocation(4, 42),
-                    // (5,24): warning CS9308: Partial property declarations 'int C.this[int p2]' and 'int C.this[int p1]' have signature differences.
+                    // (5,24): warning CS9256: Partial property declarations 'int C.this[int p2]' and 'int C.this[int p1]' have signature differences.
                     //     public partial int this[int p1] => 42;
                     Diagnostic(ErrorCode.WRN_PartialPropertySignatureDifference, "this").WithArguments("int C.this[int p2]", "int C.this[int p1]").WithLocation(5, 24));
 
@@ -2094,7 +2094,7 @@ void verify(CSharpTestSource source)
                     Assert.Equal("p2", indexer.Parameters.Single().Name);
                 });
                 verifier.VerifyDiagnostics(
-                    // (4,24): warning CS9308: Partial property declarations 'int C.this[int p2]' and 'int C.this[int p1]' have signature differences.
+                    // (4,24): warning CS9256: Partial property declarations 'int C.this[int p2]' and 'int C.this[int p1]' have signature differences.
                     //     public partial int this[int p1] => 42;
                     Diagnostic(ErrorCode.WRN_PartialPropertySignatureDifference, "this").WithArguments("int C.this[int p2]", "int C.this[int p1]").WithLocation(4, 24),
                     // (4,42): warning CS1734: XML comment on 'C.this[int]' has a paramref tag for 'p1', but there is no parameter by that name
@@ -2158,7 +2158,7 @@ void verify(CSharpTestSource source)
                     Assert.Equal("p2", indexer.Parameters.Single().Name);
                 });
                 verifier.VerifyDiagnostics(
-                    // (4,24): warning CS9308: Partial property declarations 'int C.this[int p2]' and 'int C.this[int p1]' have signature differences.
+                    // (4,24): warning CS9256: Partial property declarations 'int C.this[int p2]' and 'int C.this[int p1]' have signature differences.
                     //     public partial int this[int p1] => 42;
                     Diagnostic(ErrorCode.WRN_PartialPropertySignatureDifference, "this").WithArguments("int C.this[int p2]", "int C.this[int p1]").WithLocation(4, 24));
 
diff --git a/src/Compilers/CSharp/Test/Symbol/Symbols/PartialPropertiesTests.cs b/src/Compilers/CSharp/Test/Symbol/Symbols/PartialPropertiesTests.cs
index 74500cb2a4ff4..e44733027dd10 100644
--- a/src/Compilers/CSharp/Test/Symbol/Symbols/PartialPropertiesTests.cs
+++ b/src/Compilers/CSharp/Test/Symbol/Symbols/PartialPropertiesTests.cs
@@ -37,7 +37,7 @@ partial class C
                 """;
             var comp = CreateCompilation([source, IsExternalInitTypeDefinition]);
             comp.VerifyEmitDiagnostics(
-                // (3,17): error CS9300: Partial property 'C.P' must have an implementation part.
+                // (3,17): error CS9248: Partial property 'C.P' must have an implementation part.
                 //     partial int P { get; set; }
                 Diagnostic(ErrorCode.ERR_PartialPropertyMissingImplementation, "P").WithArguments("C.P").WithLocation(3, 17)
                 );
@@ -109,7 +109,7 @@ partial class C
                 """;
             var comp = CreateCompilation([source, IsExternalInitTypeDefinition]);
             comp.VerifyEmitDiagnostics(
-                // (3,17): error CS9301: Partial property 'C.P' must have a definition part.
+                // (3,17): error CS9249: Partial property 'C.P' must have a definition part.
                 //     partial int P { get => throw null!; set { } }
                 Diagnostic(ErrorCode.ERR_PartialPropertyMissingDefinition, "P").WithArguments("C.P").WithLocation(3, 17)
                 );
@@ -177,10 +177,10 @@ partial class C
                 """;
             var comp = CreateCompilation(source);
             comp.VerifyEmitDiagnostics(
-                // (3,17): error CS9300: Partial property 'C.P' must have an implementation part.
+                // (3,17): error CS9248: Partial property 'C.P' must have an implementation part.
                 //     partial int P { get; set; }
                 Diagnostic(ErrorCode.ERR_PartialPropertyMissingImplementation, "P").WithArguments("C.P").WithLocation(3, 17),
-                // (4,17): error CS9302: A partial property may not have multiple defining declarations, and cannot be an auto-property.
+                // (4,17): error CS9250: A partial property may not have multiple defining declarations, and cannot be an auto-property.
                 //     partial int P { get; set; }
                 Diagnostic(ErrorCode.ERR_PartialPropertyDuplicateDefinition, "P").WithLocation(4, 17),
                 // (4,17): error CS0102: The type 'C' already contains a definition for 'P'
@@ -203,7 +203,7 @@ partial class C
                 """;
             var comp = CreateCompilation(source);
             comp.VerifyEmitDiagnostics(
-                // (4,17): error CS9302: A partial property may not have multiple defining declarations, and cannot be an auto-property.
+                // (4,17): error CS9250: A partial property may not have multiple defining declarations, and cannot be an auto-property.
                 //     partial int P { get; set; }
                 Diagnostic(ErrorCode.ERR_PartialPropertyDuplicateDefinition, "P").WithLocation(4, 17),
                 // (4,17): error CS0102: The type 'C' already contains a definition for 'P'
@@ -225,10 +225,10 @@ partial class C
                 """;
             var comp = CreateCompilation(source);
             comp.VerifyEmitDiagnostics(
-                // (3,17): error CS9301: Partial property 'C.P' must have a definition part.
+                // (3,17): error CS9249: Partial property 'C.P' must have a definition part.
                 //     partial int P { get => throw null!; set { } }
                 Diagnostic(ErrorCode.ERR_PartialPropertyMissingDefinition, "P").WithArguments("C.P").WithLocation(3, 17),
-                // (4,17): error CS9303: A partial property may not have multiple implementing declarations
+                // (4,17): error CS9251: A partial property may not have multiple implementing declarations
                 //     partial int P { get => throw null!; set { } }
                 Diagnostic(ErrorCode.ERR_PartialPropertyDuplicateImplementation, "P").WithLocation(4, 17),
                 // (4,17): error CS0102: The type 'C' already contains a definition for 'P'
@@ -251,7 +251,7 @@ partial class C
                 """;
             var comp = CreateCompilation(source);
             comp.VerifyEmitDiagnostics(
-                // (5,17): error CS9303: A partial property may not have multiple implementing declarations
+                // (5,17): error CS9251: A partial property may not have multiple implementing declarations
                 //     partial int P { get => throw null!; set { } }
                 Diagnostic(ErrorCode.ERR_PartialPropertyDuplicateImplementation, "P").WithLocation(5, 17),
                 // (5,17): error CS0102: The type 'C' already contains a definition for 'P'
@@ -275,13 +275,13 @@ partial class C
                 """;
             var comp = CreateCompilation(source);
             comp.VerifyEmitDiagnostics(
-                // (4,17): error CS9302: A partial property may not have multiple defining declarations, and cannot be an auto-property.
+                // (4,17): error CS9250: A partial property may not have multiple defining declarations, and cannot be an auto-property.
                 //     partial int P { get; set; }
                 Diagnostic(ErrorCode.ERR_PartialPropertyDuplicateDefinition, "P").WithLocation(4, 17),
                 // (4,17): error CS0102: The type 'C' already contains a definition for 'P'
                 //     partial int P { get; set; }
                 Diagnostic(ErrorCode.ERR_DuplicateNameInClass, "P").WithArguments("C", "P").WithLocation(4, 17),
-                // (6,17): error CS9303: A partial property may not have multiple implementing declarations
+                // (6,17): error CS9251: A partial property may not have multiple implementing declarations
                 //     partial int P { get => throw null!; set { } }
                 Diagnostic(ErrorCode.ERR_PartialPropertyDuplicateImplementation, "P").WithLocation(6, 17),
                 // (6,17): error CS0102: The type 'C' already contains a definition for 'P'
@@ -303,7 +303,7 @@ partial class C
                 """;
             var comp = CreateCompilation(source);
             comp.VerifyEmitDiagnostics(
-                // (3,24): error CS9300: Partial property 'C.P' must have an implementation part.
+                // (3,24): error CS9248: Partial property 'C.P' must have an implementation part.
                 //     public partial int P { get; set; }
                 Diagnostic(ErrorCode.ERR_PartialPropertyMissingImplementation, "P").WithArguments("C.P").WithLocation(3, 24),
                 // (4,24): error CS0759: No defining declaration found for implementing declaration of partial method 'C.P()'
@@ -328,7 +328,7 @@ partial class C
                 """;
             var comp = CreateCompilation(source);
             comp.VerifyEmitDiagnostics(
-                // (3,24): error CS9300: Partial property 'C.P' must have an implementation part.
+                // (3,24): error CS9248: Partial property 'C.P' must have an implementation part.
                 //     public partial int P { get; }
                 Diagnostic(ErrorCode.ERR_PartialPropertyMissingImplementation, "P").WithArguments("C.P").WithLocation(3, 24),
                 // (3,28): error CS0082: Type 'C' already reserves a member called 'get_P' with the same parameter types
@@ -351,10 +351,10 @@ public partial int P { set { } }
                 """;
             var comp = CreateCompilation(source);
             comp.VerifyEmitDiagnostics(
-                // (4,24): error CS9304: Property accessor 'C.P.set' must be implemented because it is declared on the definition part
+                // (4,24): error CS9252: Property accessor 'C.P.set' must be implemented because it is declared on the definition part
                 //     public partial int P { get => 1; }
                 Diagnostic(ErrorCode.ERR_PartialPropertyMissingAccessor, "P").WithArguments("C.P.set").WithLocation(4, 24),
-                // (5,24): error CS9303: A partial property may not have multiple implementing declarations
+                // (5,24): error CS9251: A partial property may not have multiple implementing declarations
                 //     public partial int P { set { } }
                 Diagnostic(ErrorCode.ERR_PartialPropertyDuplicateImplementation, "P").WithLocation(5, 24),
                 // (5,24): error CS0102: The type 'C' already contains a definition for 'P'
@@ -374,6 +374,43 @@ public partial int P { set { } }
             Assert.Equal("System.Int32 C.P { set; }", duplicateProp.ToTestDisplayString());
         }
 
+        [Fact]
+        public void DuplicateDeclaration_08_Definition()
+        {
+            // multiple defining declarations where accessors are "split" across declarations
+            var source = """
+                partial class C
+                {
+                    public partial int P { get; }
+                    public partial int P { set; }
+                    public partial int P { get => 1; set { } }
+                }
+                """;
+            var comp = CreateCompilation(source);
+            comp.VerifyEmitDiagnostics(
+                // (4,24): error CS9250: A partial property may not have multiple defining declarations, and cannot be an auto-property.
+                //     public partial int P { set; }
+                Diagnostic(ErrorCode.ERR_PartialPropertyDuplicateDefinition, "P").WithLocation(4, 24),
+                // (4,24): error CS0102: The type 'C' already contains a definition for 'P'
+                //     public partial int P { set; }
+                Diagnostic(ErrorCode.ERR_DuplicateNameInClass, "P").WithArguments("C", "P").WithLocation(4, 24),
+                // (5,24): error CS9253: Property accessor 'C.P.set' does not implement any accessor declared on the definition part
+                //     public partial int P { get => 1; set { } }
+                Diagnostic(ErrorCode.ERR_PartialPropertyUnexpectedAccessor, "P").WithArguments("C.P.set").WithLocation(5, 24)
+                );
+
+            if (comp.GetMembers("C.P") is not [SourcePropertySymbol prop, SourcePropertySymbol duplicateProp])
+                throw ExceptionUtilities.UnexpectedValue(comp.GetMembers("C.P"));
+
+            Assert.True(prop.IsPartialDefinition);
+            Assert.Equal("System.Int32 C.P { get; }", prop.ToTestDisplayString());
+            Assert.Equal("System.Int32 C.P { get; set; }", prop.PartialImplementationPart.ToTestDisplayString());
+
+            Assert.True(duplicateProp.IsPartialDefinition);
+            Assert.Null(duplicateProp.PartialImplementationPart);
+            Assert.Equal("System.Int32 C.P { set; }", duplicateProp.ToTestDisplayString());
+        }
+
         [Fact]
         public void DuplicateDeclaration_09()
         {
@@ -387,13 +424,13 @@ partial class C
                 """;
             var comp = CreateCompilation(source);
             comp.VerifyEmitDiagnostics(
-                // (3,24): error CS9300: Partial property 'C.this[int]' must have an implementation part.
+                // (3,24): error CS9248: Partial property 'C.this[int]' must have an implementation part.
                 //     public partial int this[int i] { get; }
                 Diagnostic(ErrorCode.ERR_PartialPropertyMissingImplementation, "this").WithArguments("C.this[int]").WithLocation(3, 24),
                 // (3,24): error CS0102: The type 'C' already contains a definition for 'Item'
                 //     public partial int this[int i] { get; }
                 Diagnostic(ErrorCode.ERR_DuplicateNameInClass, "this").WithArguments("C", "Item").WithLocation(3, 24),
-                // (4,24): error CS9301: Partial property 'C.Item' must have a definition part.
+                // (4,24): error CS9249: Partial property 'C.Item' must have a definition part.
                 //     public partial int Item => 1;
                 Diagnostic(ErrorCode.ERR_PartialPropertyMissingDefinition, "Item").WithArguments("C.Item").WithLocation(4, 24)
                 );
@@ -412,7 +449,7 @@ partial class C
                 """;
             var comp = CreateCompilation(source);
             comp.VerifyEmitDiagnostics(
-                // (3,24): error CS9300: Partial property 'C.this' must have an implementation part.
+                // (3,24): error CS9248: Partial property 'C.this' must have an implementation part.
                 //     public partial int this[] { get; }
                 Diagnostic(ErrorCode.ERR_PartialPropertyMissingImplementation, "this").WithArguments("C.this").WithLocation(3, 24),
                 // (3,24): error CS0102: The type 'C' already contains a definition for 'Item'
@@ -421,7 +458,7 @@ partial class C
                 // (3,29): error CS1551: Indexers must have at least one parameter
                 //     public partial int this[] { get; }
                 Diagnostic(ErrorCode.ERR_IndexerNeedsParam, "]").WithLocation(3, 29),
-                // (4,24): error CS9301: Partial property 'C.Item' must have a definition part.
+                // (4,24): error CS9249: Partial property 'C.Item' must have a definition part.
                 //     public partial int Item => 1;
                 Diagnostic(ErrorCode.ERR_PartialPropertyMissingDefinition, "Item").WithArguments("C.Item").WithLocation(4, 24)
                 );
@@ -440,7 +477,7 @@ partial class C
                 """;
             var comp = CreateCompilation(source);
             comp.VerifyEmitDiagnostics(
-                // (4,17): error CS9304: Property accessor 'C.P.set' must be implemented because it is declared on the definition part
+                // (4,17): error CS9252: Property accessor 'C.P.set' must be implemented because it is declared on the definition part
                 //     partial int P { get => throw null!; }
                 Diagnostic(ErrorCode.ERR_PartialPropertyMissingAccessor, "P").WithArguments("C.P.set").WithLocation(4, 17)
                 );
@@ -459,7 +496,7 @@ partial int P { set { } }
                 """;
             var comp = CreateCompilation(source);
             comp.VerifyEmitDiagnostics(
-                // (4,17): error CS9304: Property accessor 'C.P.get' must be implemented because it is declared on the definition part
+                // (4,17): error CS9252: Property accessor 'C.P.get' must be implemented because it is declared on the definition part
                 //     partial int P { set { } }
                 Diagnostic(ErrorCode.ERR_PartialPropertyMissingAccessor, "P").WithArguments("C.P.get").WithLocation(4, 17)
                 );
@@ -478,7 +515,7 @@ partial class C
                 """;
             var comp = CreateCompilation([source, IsExternalInitTypeDefinition]);
             comp.VerifyEmitDiagnostics(
-                // (4,17): error CS9304: Property accessor 'C.P.init' must be implemented because it is declared on the definition part
+                // (4,17): error CS9252: Property accessor 'C.P.init' must be implemented because it is declared on the definition part
                 //     partial int P { get => throw null!; }
                 Diagnostic(ErrorCode.ERR_PartialPropertyMissingAccessor, "P").WithArguments("C.P.init").WithLocation(4, 17)
                 );
@@ -500,10 +537,10 @@ partial int P { }
                 """;
             var comp = CreateCompilation([source, IsExternalInitTypeDefinition]);
             comp.VerifyEmitDiagnostics(
-                // (3,17): error CS9300: Partial property 'C.P' must have an implementation part.
+                // (3,17): error CS9248: Partial property 'C.P' must have an implementation part.
                 //     partial int P { {{accessorKind}}; }
                 Diagnostic(ErrorCode.ERR_PartialPropertyMissingImplementation, "P").WithArguments("C.P").WithLocation(3, 17),
-                // (4,17): error CS9302: A partial property may not have multiple defining declarations, and cannot be an auto-property.
+                // (4,17): error CS9250: A partial property may not have multiple defining declarations, and cannot be an auto-property.
                 //     partial int P { }
                 Diagnostic(ErrorCode.ERR_PartialPropertyDuplicateDefinition, "P").WithLocation(4, 17),
                 // (4,17): error CS0102: The type 'C' already contains a definition for 'P'
@@ -531,7 +568,7 @@ partial int P { }
                 """;
             var comp = CreateCompilation([source, IsExternalInitTypeDefinition]);
             comp.VerifyEmitDiagnostics(
-                // (3,17): error CS9305: Property accessor 'C.P.{accessorKind}' does not implement any accessor declared on the definition part
+                // (3,17): error CS9253: Property accessor 'C.P.{accessorKind}' does not implement any accessor declared on the definition part
                 //     partial int P { {{accessorKind}} => throw null!; }
                 Diagnostic(ErrorCode.ERR_PartialPropertyUnexpectedAccessor, "P").WithArguments($"C.P.{accessorKind}").WithLocation(3, 17),
                 // (4,17): error CS0548: 'C.P': property or indexer must have at least one accessor
@@ -553,7 +590,7 @@ partial class C
                 """;
             var comp = CreateCompilation(source);
             comp.VerifyEmitDiagnostics(
-                // (4,17): error CS9305: Property accessor 'C.P.set' does not implement any accessor declared on the definition part
+                // (4,17): error CS9253: Property accessor 'C.P.set' does not implement any accessor declared on the definition part
                 //     partial int P { get => throw null!; set { } }
                 Diagnostic(ErrorCode.ERR_PartialPropertyUnexpectedAccessor, "P").WithArguments("C.P.set").WithLocation(4, 17)
                 );
@@ -572,7 +609,7 @@ partial class C
                 """;
             var comp = CreateCompilation(source);
             comp.VerifyEmitDiagnostics(
-                // (4,17): error CS9305: Property accessor 'C.P.get' does not implement any accessor declared on the definition part
+                // (4,17): error CS9253: Property accessor 'C.P.get' does not implement any accessor declared on the definition part
                 //     partial int P { get => throw null!; set { } }
                 Diagnostic(ErrorCode.ERR_PartialPropertyUnexpectedAccessor, "P").WithArguments("C.P.get").WithLocation(4, 17)
                 );
@@ -591,7 +628,7 @@ partial class C
                 """;
             var comp = CreateCompilation([source, IsExternalInitTypeDefinition]);
             comp.VerifyEmitDiagnostics(
-                // (4,17): error CS9305: Property accessor 'C.P.init' does not implement any accessor declared on the definition part
+                // (4,17): error CS9253: Property accessor 'C.P.init' does not implement any accessor declared on the definition part
                 //     partial int P { get => throw null!; init { } }
                 Diagnostic(ErrorCode.ERR_PartialPropertyUnexpectedAccessor, "P").WithArguments("C.P.init").WithLocation(4, 17)
                 );
@@ -611,7 +648,7 @@ partial class C
 
             var comp = CreateCompilation([source, IsExternalInitTypeDefinition]);
             comp.VerifyEmitDiagnostics(
-                // (4,41): error CS9306: Property accessor 'C.P.init' must be 'set' to match the definition part       
+                // (4,41): error CS9254: Property accessor 'C.P.init' must be 'set' to match the definition part       
                 //     partial int P { get => throw null!; init { } }
                 Diagnostic(ErrorCode.ERR_PartialPropertyInitMismatch, "init").WithArguments("C.P.init", "set").WithLocation(4, 41)
                 );
@@ -630,7 +667,7 @@ partial class C
                 """;
             var comp = CreateCompilation([source, IsExternalInitTypeDefinition]);
             comp.VerifyEmitDiagnostics(
-                // (4,41): error CS9306: Property accessor 'C.P.set' must be 'init' to match the definition part
+                // (4,41): error CS9254: Property accessor 'C.P.set' must be 'init' to match the definition part
                 //     partial int P { get => throw null!; set { } }
                 Diagnostic(ErrorCode.ERR_PartialPropertyInitMismatch, "set").WithArguments("C.P.set", "init").WithLocation(4, 41)
                 );
@@ -689,10 +726,10 @@ partial class C
                 """;
             var comp = CreateCompilation([source, IsExternalInitTypeDefinition]);
             comp.VerifyEmitDiagnostics(
-                // (3,24): error CS9301: Partial property 'C.P' must have a definition part.
+                // (3,24): error CS9249: Partial property 'C.P' must have a definition part.
                 //     extern partial int P { get; set; }
                 Diagnostic(ErrorCode.ERR_PartialPropertyMissingDefinition, "P").WithArguments("C.P").WithLocation(3, 24),
-                // (4,24): error CS9303: A partial property may not have multiple implementing declarations
+                // (4,24): error CS9251: A partial property may not have multiple implementing declarations
                 //     extern partial int P { get; set; }
                 Diagnostic(ErrorCode.ERR_PartialPropertyDuplicateImplementation, "P").WithLocation(4, 24),
                 // (4,24): error CS0102: The type 'C' already contains a definition for 'P'
@@ -1512,13 +1549,13 @@ partial class C
                 """;
             var comp = CreateCompilation(source);
             comp.VerifyEmitDiagnostics(
-                // (6,20): error CS9307: Both partial property declarations must have the same type.
+                // (6,20): error CS9255: Both partial property declarations must have the same type.
                 //     partial string P1 { get => ""; set { } }
                 Diagnostic(ErrorCode.ERR_PartialPropertyTypeDifference, "P1").WithLocation(6, 20),
-                // (9,26): error CS9307: Both partial property declarations must have the same type.
+                // (9,26): error CS9255: Both partial property declarations must have the same type.
                 //     partial List<string> P2 { get => []; set { } }
                 Diagnostic(ErrorCode.ERR_PartialPropertyTypeDifference, "P2").WithLocation(9, 26),
-                // (12,33): error CS9307: Both partial property declarations must have the same type.
+                // (12,33): error CS9255: Both partial property declarations must have the same type.
                 //     partial IEnumerable<string> P3 { get => []; set { } }
                 Diagnostic(ErrorCode.ERR_PartialPropertyTypeDifference, "P3").WithLocation(12, 33));
         }
@@ -1545,16 +1582,16 @@ partial class C
                 """;
             var comp = CreateCompilation(source);
             comp.VerifyEmitDiagnostics(
-                // (5,20): error CS9308: Partial property declarations 'string? C.P1' and 'string C.P1' have signature differences.
+                // (5,20): error CS9256: Partial property declarations 'string? C.P1' and 'string C.P1' have signature differences.
                 //     partial string P1 { get => ""; set { } }
                 Diagnostic(ErrorCode.WRN_PartialPropertySignatureDifference, "P1").WithArguments("string? C.P1", "string C.P1").WithLocation(5, 20),
-                // (8,21): error CS9308: Partial property declarations 'string C.P2' and 'string? C.P2' have signature differences.
+                // (8,21): error CS9256: Partial property declarations 'string C.P2' and 'string? C.P2' have signature differences.
                 //     partial string? P2 { get => ""; set { } }
                 Diagnostic(ErrorCode.WRN_PartialPropertySignatureDifference, "P2").WithArguments("string C.P2", "string? C.P2").WithLocation(8, 21),
-                // (11,22): error CS9308: Partial property declarations 'string?[] C.P3' and 'string[] C.P3' have signature differences.
+                // (11,22): error CS9256: Partial property declarations 'string?[] C.P3' and 'string[] C.P3' have signature differences.
                 //     partial string[] P3 { get => []; set { } }
                 Diagnostic(ErrorCode.WRN_PartialPropertySignatureDifference, "P3").WithArguments("string?[] C.P3", "string[] C.P3").WithLocation(11, 22),
-                // (14,23): error CS9308: Partial property declarations 'string[] C.P4' and 'string?[] C.P4' have signature differences.
+                // (14,23): error CS9256: Partial property declarations 'string[] C.P4' and 'string?[] C.P4' have signature differences.
                 //     partial string?[] P4 { get => []; set { } }
                 Diagnostic(ErrorCode.WRN_PartialPropertySignatureDifference, "P4").WithArguments("string[] C.P4", "string?[] C.P4").WithLocation(14, 23));
         }
@@ -1583,10 +1620,10 @@ partial class C
 
             var comp = CreateCompilation(source, options: TestOptions.DebugDll.WithNullableContextOptions(NullableContextOptions.Enable));
             comp.VerifyEmitDiagnostics(
-                // (4,27): warning CS9308: Partial property declarations 'string? C.P1' and 'string C.P1' have signature differences.
+                // (4,27): warning CS9256: Partial property declarations 'string? C.P1' and 'string C.P1' have signature differences.
                 //     public partial string P1 { get => ""; set { } }
                 Diagnostic(ErrorCode.WRN_PartialPropertySignatureDifference, "P1").WithArguments("string? C.P1", "string C.P1").WithLocation(4, 27),
-                // (7,28): warning CS9308: Partial property declarations 'string C.P2' and 'string? C.P2' have signature differences.
+                // (7,28): warning CS9256: Partial property declarations 'string C.P2' and 'string? C.P2' have signature differences.
                 //     public partial string? P2 { get => ""; set { } }
                 Diagnostic(ErrorCode.WRN_PartialPropertySignatureDifference, "P2").WithArguments("string C.P2", "string? C.P2").WithLocation(7, 28),
                 // (10,27): warning CS8826: Partial method declarations 'string? C.M1()' and 'string C.M1()' have signature differences.
@@ -1598,10 +1635,10 @@ partial class C
 
             comp = CreateCompilation(source, options: TestOptions.DebugDll.WithNullableContextOptions(NullableContextOptions.Annotations));
             comp.VerifyEmitDiagnostics(
-                // (4,27): warning CS9308: Partial property declarations 'string? C.P1' and 'string C.P1' have signature differences.
+                // (4,27): warning CS9256: Partial property declarations 'string? C.P1' and 'string C.P1' have signature differences.
                 //     public partial string P1 { get => ""; set { } }
                 Diagnostic(ErrorCode.WRN_PartialPropertySignatureDifference, "P1").WithArguments("string? C.P1", "string C.P1").WithLocation(4, 27),
-                // (7,28): warning CS9308: Partial property declarations 'string C.P2' and 'string? C.P2' have signature differences.
+                // (7,28): warning CS9256: Partial property declarations 'string C.P2' and 'string? C.P2' have signature differences.
                 //     public partial string? P2 { get => ""; set { } }
                 Diagnostic(ErrorCode.WRN_PartialPropertySignatureDifference, "P2").WithArguments("string C.P2", "string? C.P2").WithLocation(7, 28),
                 // (10,27): warning CS8826: Partial method declarations 'string? C.M1()' and 'string C.M1()' have signature differences.
@@ -1648,6 +1685,91 @@ partial class C<T>
             Assert.Equal(NullableAnnotation.Oblivious, p3.TypeWithAnnotations.NullableAnnotation);
         }
 
+        [Fact]
+        public void NullableDifference_Analysis()
+        {
+            // The implementation part signature is used to analyze the implementation part bodies.
+            // The definition part signature is used to analyze use sites.
+            // This is consistent with methods.
+            var source = """
+                #nullable enable
+                partial class C
+                {
+                    public partial string this[string? x] { get; set; }
+                    public partial string? this[string x] // 1
+                    {
+                        get => x.ToString();
+                        set => x.ToString();
+                    }
+                    
+                    public partial string? this[string x, bool ignored] { get; set; }
+                    public partial string this[string? x, bool ignored] // 2
+                    {
+                        get => x.ToString(); // 3
+                        set => x.ToString(); // 4
+                    }
+                
+                    public partial string this[bool ignored] { get; }
+                    public partial string? this[bool ignored] // 5
+                    {
+                        get => null;
+                    }
+                
+                    public partial string? this[int ignored] { get; }
+                    public partial string this[int ignored] // 6
+                    {
+                        get => null; // 7
+                    }
+
+                    void Usage()
+                    {
+                        this[x: null].ToString();
+                        this[x: null, ignored: false].ToString(); // 8, 9
+                    }
+                }
+                """;
+
+            var verifier = CompileAndVerify(source, symbolValidator: verify, sourceSymbolValidator: verify);
+            verifier.VerifyDiagnostics(
+                // (5,28): warning CS9256: Partial property declarations 'string C.this[string? x]' and 'string? C.this[string x]' have signature differences.
+                //     public partial string? this[string x] // 1
+                Diagnostic(ErrorCode.WRN_PartialPropertySignatureDifference, "this").WithArguments("string C.this[string? x]", "string? C.this[string x]").WithLocation(5, 28),
+                // (12,27): warning CS9256: Partial property declarations 'string? C.this[string x, bool ignored]' and 'string C.this[string? x, bool ignored]' have signature differences.
+                //     public partial string this[string? x, bool ignored] // 2
+                Diagnostic(ErrorCode.WRN_PartialPropertySignatureDifference, "this").WithArguments("string? C.this[string x, bool ignored]", "string C.this[string? x, bool ignored]").WithLocation(12, 27),
+                // (14,16): warning CS8602: Dereference of a possibly null reference.
+                //         get => x.ToString(); // 3
+                Diagnostic(ErrorCode.WRN_NullReferenceReceiver, "x").WithLocation(14, 16),
+                // (15,16): warning CS8602: Dereference of a possibly null reference.
+                //         set => x.ToString(); // 4
+                Diagnostic(ErrorCode.WRN_NullReferenceReceiver, "x").WithLocation(15, 16),
+                // (19,28): warning CS9256: Partial property declarations 'string C.this[bool ignored]' and 'string? C.this[bool ignored]' have signature differences.
+                //     public partial string? this[bool ignored] // 5
+                Diagnostic(ErrorCode.WRN_PartialPropertySignatureDifference, "this").WithArguments("string C.this[bool ignored]", "string? C.this[bool ignored]").WithLocation(19, 28),
+                // (25,27): warning CS9256: Partial property declarations 'string? C.this[int ignored]' and 'string C.this[int ignored]' have signature differences.
+                //     public partial string this[int ignored] // 6
+                Diagnostic(ErrorCode.WRN_PartialPropertySignatureDifference, "this").WithArguments("string? C.this[int ignored]", "string C.this[int ignored]").WithLocation(25, 27),
+                // (27,16): warning CS8603: Possible null reference return.
+                //         get => null; // 7
+                Diagnostic(ErrorCode.WRN_NullReferenceReturn, "null").WithLocation(27, 16),
+                // (33,9): warning CS8602: Dereference of a possibly null reference.
+                //         this[x: null, ignored: false].ToString(); // 8, 9
+                Diagnostic(ErrorCode.WRN_NullReferenceReceiver, "this[x: null, ignored: false]").WithLocation(33, 9),
+                // (33,17): warning CS8625: Cannot convert null literal to non-nullable reference type.
+                //         this[x: null, ignored: false].ToString(); // 8, 9
+                Diagnostic(ErrorCode.WRN_NullAsNonNullable, "null").WithLocation(33, 17));
+
+            void verify(ModuleSymbol module)
+            {
+                var indexers = module.GlobalNamespace.GetMember<NamedTypeSymbol>("C").Indexers;
+                Assert.Equal(4, indexers.Length);
+                AssertEx.Equal("System.String C.this[System.String? x] { get; set; }", indexers[0].ToTestDisplayString());
+                AssertEx.Equal("System.String? C.this[System.String x, System.Boolean ignored] { get; set; }", indexers[1].ToTestDisplayString());
+                AssertEx.Equal("System.String C.this[System.Boolean ignored] { get; }", indexers[2].ToTestDisplayString());
+                AssertEx.Equal("System.String? C.this[System.Int32 ignored] { get; }", indexers[3].ToTestDisplayString());
+            }
+        }
+
         [Fact]
         public void TypeDifference_03()
         {
@@ -1788,7 +1910,7 @@ partial class C
 
             var comp = CreateCompilation(source);
             comp.VerifyEmitDiagnostics(
-                // (6,52): error CS9307: Both partial property declarations must have the same type.
+                // (6,52): error CS9255: Both partial property declarations must have the same type.
                 //     public partial ref readonly (long x, string y) Prop => throw null!;
                 Diagnostic(ErrorCode.ERR_PartialPropertyTypeDifference, "Prop").WithLocation(6, 52),
                 // (6,52): error CS8818: Partial member declarations must have matching ref return values.
@@ -2134,13 +2256,13 @@ abstract partial class C
 
             var comp = CreateCompilation(source);
             comp.VerifyEmitDiagnostics(
-                // (3,33): error CS9300: Partial property 'C.P1' must have an implementation part.
+                // (3,33): error CS9248: Partial property 'C.P1' must have an implementation part.
                 //     public abstract partial int P1 { get; set; }
                 Diagnostic(ErrorCode.ERR_PartialPropertyMissingImplementation, "P1").WithArguments("C.P1").WithLocation(3, 33),
                 // (3,33): error CS0750: A partial member cannot have the 'abstract' modifier
                 //     public abstract partial int P1 { get; set; }
                 Diagnostic(ErrorCode.ERR_PartialMemberCannotBeAbstract, "P1").WithLocation(3, 33),
-                // (5,33): error CS9301: Partial property 'C.P2' must have a definition part.
+                // (5,33): error CS9249: Partial property 'C.P2' must have a definition part.
                 //     public abstract partial int P2 { get => ""; set { } }
                 Diagnostic(ErrorCode.ERR_PartialPropertyMissingDefinition, "P2").WithArguments("C.P2").WithLocation(5, 33),
                 // (5,33): error CS0750: A partial member cannot have the 'abstract' modifier
@@ -2251,10 +2373,10 @@ static void Main()
 
             var comp = CreateCompilation([source, RequiredMemberAttribute, SetsRequiredMembersAttribute, CompilerFeatureRequiredAttribute]);
             comp.VerifyEmitDiagnostics(
-                // (4,27): error CS9309: Both partial property declarations must be required or neither may be required
+                // (4,27): error CS9257: Both partial property declarations must be required or neither may be required
                 //     public partial string P1 { get => ""; set { } }
                 Diagnostic(ErrorCode.ERR_PartialPropertyRequiredDifference, "P1").WithLocation(4, 27),
-                // (7,36): error CS9309: Both partial property declarations must be required or neither may be required
+                // (7,36): error CS9257: Both partial property declarations must be required or neither may be required
                 //     public required partial string P2 { get => ""; set { } }
                 Diagnostic(ErrorCode.ERR_PartialPropertyRequiredDifference, "P2").WithLocation(7, 36),
                 // (11,17): error CS9035: Required member 'C.P1' must be set in the object initializer or attribute constructor.
@@ -2286,7 +2408,7 @@ partial class C
 
             var comp = CreateCompilation(source);
             comp.VerifyEmitDiagnostics(
-                // (15,26): error CS9307: Both partial property declarations must have the same type.
+                // (15,26): error CS9255: Both partial property declarations must have the same type.
                 //     public partial MyInt P3 { get => 3; set { } }
                 Diagnostic(ErrorCode.ERR_PartialPropertyTypeDifference, "P3").WithLocation(15, 26));
         }
@@ -2322,13 +2444,13 @@ partial class C3 : I
                 // (8,19): error CS0754: A partial member may not explicitly implement an interface member
                 //     partial int I.P { get; set; }
                 Diagnostic(ErrorCode.ERR_PartialMemberNotExplicit, "P").WithLocation(8, 19),
-                // (14,19): error CS9300: Partial property 'C2.I.P' must have an implementation part.
+                // (14,19): error CS9248: Partial property 'C2.I.P' must have an implementation part.
                 //     partial int I.P { get; set; }
                 Diagnostic(ErrorCode.ERR_PartialPropertyMissingImplementation, "P").WithArguments("C2.I.P").WithLocation(14, 19),
                 // (14,19): error CS0754: A partial member may not explicitly implement an interface member
                 //     partial int I.P { get; set; }
                 Diagnostic(ErrorCode.ERR_PartialMemberNotExplicit, "P").WithLocation(14, 19),
-                // (19,19): error CS9301: Partial property 'C3.I.P' must have a definition part.
+                // (19,19): error CS9249: Partial property 'C3.I.P' must have a definition part.
                 //     partial int I.P { get => 1; set { } }
                 Diagnostic(ErrorCode.ERR_PartialPropertyMissingDefinition, "P").WithArguments("C3.I.P").WithLocation(19, 19),
                 // (19,19): error CS0754: A partial member may not explicitly implement an interface member
@@ -2356,13 +2478,13 @@ class C
                 // (3,17): error CS0751: A partial member must be declared within a partial type
                 //     partial int P1 { get; set; }
                 Diagnostic(ErrorCode.ERR_PartialMemberOnlyInPartialClass, "P1").WithLocation(3, 17),
-                // (6,17): error CS9300: Partial property 'C.P2' must have an implementation part.
+                // (6,17): error CS9248: Partial property 'C.P2' must have an implementation part.
                 //     partial int P2 { get; set; }
                 Diagnostic(ErrorCode.ERR_PartialPropertyMissingImplementation, "P2").WithArguments("C.P2").WithLocation(6, 17),
                 // (6,17): error CS0751: A partial member must be declared within a partial type
                 //     partial int P2 { get; set; }
                 Diagnostic(ErrorCode.ERR_PartialMemberOnlyInPartialClass, "P2").WithLocation(6, 17),
-                // (8,17): error CS9301: Partial property 'C.P3' must have a definition part.
+                // (8,17): error CS9249: Partial property 'C.P3' must have a definition part.
                 //     partial int P3 { get => 1; set { } }
                 Diagnostic(ErrorCode.ERR_PartialPropertyMissingDefinition, "P3").WithArguments("C.P3").WithLocation(8, 17),
                 // (8,17): error CS0751: A partial member must be declared within a partial type
@@ -2423,22 +2545,22 @@ partial class C2
 
             var comp = CreateCompilation(source);
             comp.VerifyEmitDiagnostics(
-                // (3,24): error CS9300: Partial property 'C1.this[int]' must have an implementation part.
+                // (3,24): error CS9248: Partial property 'C1.this[int]' must have an implementation part.
                 //     public partial int this[int i] { get; set; }
                 Diagnostic(ErrorCode.ERR_PartialPropertyMissingImplementation, "this").WithArguments("C1.this[int]").WithLocation(3, 24),
-                // (4,24): error CS9301: Partial property 'C1.this[ref int]' must have a definition part.
+                // (4,24): error CS9249: Partial property 'C1.this[ref int]' must have a definition part.
                 //     public partial int this[ref int i] { get => i; set { } }
                 Diagnostic(ErrorCode.ERR_PartialPropertyMissingDefinition, "this").WithArguments($"C1.this[{refKind} int]").WithLocation(4, 24),
                 // (4,29): error CS0631: ref and out are not valid in this context
                 //     public partial int this[ref int i] { get => i; set { } }
                 Diagnostic(ErrorCode.ERR_IllegalRefParam, refKind).WithLocation(4, 29),
-                // (9,24): error CS9300: Partial property 'C2.this[ref int]' must have an implementation part.
+                // (9,24): error CS9248: Partial property 'C2.this[ref int]' must have an implementation part.
                 //     public partial int this[ref int i] { get; set; }
                 Diagnostic(ErrorCode.ERR_PartialPropertyMissingImplementation, "this").WithArguments($"C2.this[{refKind} int]").WithLocation(9, 24),
                 // (9,29): error CS0631: ref and out are not valid in this context
                 //     public partial int this[ref int i] { get; set; }
                 Diagnostic(ErrorCode.ERR_IllegalRefParam, refKind).WithLocation(9, 29),
-                // (10,24): error CS9301: Partial property 'C2.this[int]' must have a definition part.
+                // (10,24): error CS9249: Partial property 'C2.this[int]' must have a definition part.
                 //     public partial int this[int i] { get => i; set { } }
                 Diagnostic(ErrorCode.ERR_PartialPropertyMissingDefinition, "this").WithArguments("C2.this[int]").WithLocation(10, 24));
         }
@@ -2464,16 +2586,16 @@ partial class C2
 
             var comp = CreateCompilation(source);
             comp.VerifyEmitDiagnostics(
-                // (3,24): error CS9300: Partial property 'C1.this[int]' must have an implementation part.
+                // (3,24): error CS9248: Partial property 'C1.this[int]' must have an implementation part.
                 //     public partial int this[int i] { get; set; }
                 Diagnostic(ErrorCode.ERR_PartialPropertyMissingImplementation, "this").WithArguments("C1.this[int]").WithLocation(3, 24),
-                // (4,24): error CS9301: Partial property 'C1.this[in int]' must have a definition part.
+                // (4,24): error CS9249: Partial property 'C1.this[in int]' must have a definition part.
                 //     public partial int this[in int i] { get => i; set { } }
                 Diagnostic(ErrorCode.ERR_PartialPropertyMissingDefinition, "this").WithArguments($"C1.this[{refKind} int]").WithLocation(4, 24),
-                // (9,24): error CS9300: Partial property 'C2.this[in int]' must have an implementation part.
+                // (9,24): error CS9248: Partial property 'C2.this[in int]' must have an implementation part.
                 //     public partial int this[in int i] { get; set; }
                 Diagnostic(ErrorCode.ERR_PartialPropertyMissingImplementation, "this").WithArguments($"C2.this[{refKind} int]").WithLocation(9, 24),
-                // (10,24): error CS9301: Partial property 'C2.this[int]' must have a definition part.
+                // (10,24): error CS9249: Partial property 'C2.this[int]' must have a definition part.
                 //     public partial int this[int i] { get => i; set { } }
                 Diagnostic(ErrorCode.ERR_PartialPropertyMissingDefinition, "this").WithArguments("C2.this[int]").WithLocation(10, 24));
         }
@@ -2497,13 +2619,13 @@ partial class C2
 
             var comp = CreateCompilation(source);
             comp.VerifyEmitDiagnostics(
-                // (3,24): error CS9300: Partial property 'C1.this[ref int]' must have an implementation part.
+                // (3,24): error CS9248: Partial property 'C1.this[ref int]' must have an implementation part.
                 //     public partial int this[ref int i] { get; set; }
                 Diagnostic(ErrorCode.ERR_PartialPropertyMissingImplementation, "this").WithArguments("C1.this[ref int]").WithLocation(3, 24),
                 // (3,29): error CS0631: ref and out are not valid in this context
                 //     public partial int this[ref int i] { get; set; }
                 Diagnostic(ErrorCode.ERR_IllegalRefParam, "ref").WithLocation(3, 29),
-                // (4,24): error CS9301: Partial property 'C1.this[out int]' must have a definition part.
+                // (4,24): error CS9249: Partial property 'C1.this[out int]' must have a definition part.
                 //     public partial int this[out int i] { get => i = 0; set => i = 0; }
                 Diagnostic(ErrorCode.ERR_PartialPropertyMissingDefinition, "this").WithArguments("C1.this[out int]").WithLocation(4, 24),
                 // (4,24): error CS0111: Type 'C1' already defines a member called 'this' with the same parameter types
@@ -2512,13 +2634,13 @@ partial class C2
                 // (4,29): error CS0631: ref and out are not valid in this context
                 //     public partial int this[out int i] { get => i = 0; set => i = 0; }
                 Diagnostic(ErrorCode.ERR_IllegalRefParam, "out").WithLocation(4, 29),
-                // (9,24): error CS9300: Partial property 'C2.this[out int]' must have an implementation part.
+                // (9,24): error CS9248: Partial property 'C2.this[out int]' must have an implementation part.
                 //     public partial int this[out int i] { get; set; }
                 Diagnostic(ErrorCode.ERR_PartialPropertyMissingImplementation, "this").WithArguments("C2.this[out int]").WithLocation(9, 24),
                 // (9,29): error CS0631: ref and out are not valid in this context
                 //     public partial int this[out int i] { get; set; }
                 Diagnostic(ErrorCode.ERR_IllegalRefParam, "out").WithLocation(9, 29),
-                // (10,24): error CS9301: Partial property 'C2.this[ref int]' must have a definition part.
+                // (10,24): error CS9249: Partial property 'C2.this[ref int]' must have a definition part.
                 //     public partial int this[ref int i] { get => i; set { } }
                 Diagnostic(ErrorCode.ERR_PartialPropertyMissingDefinition, "this").WithArguments("C2.this[ref int]").WithLocation(10, 24),
                 // (10,24): error CS0111: Type 'C2' already defines a member called 'this' with the same parameter types
@@ -2549,19 +2671,19 @@ partial class C2
 
             var comp = CreateCompilation(source);
             comp.VerifyEmitDiagnostics(
-                // (3,24): error CS9300: Partial property 'C1.this[in int]' must have an implementation part.
+                // (3,24): error CS9248: Partial property 'C1.this[in int]' must have an implementation part.
                 //     public partial int this[in int i] { get; set; }
                 Diagnostic(ErrorCode.ERR_PartialPropertyMissingImplementation, "this").WithArguments("C1.this[in int]").WithLocation(3, 24),
-                // (4,24): error CS9301: Partial property 'C1.this[ref readonly int]' must have a definition part.
+                // (4,24): error CS9249: Partial property 'C1.this[ref readonly int]' must have a definition part.
                 //     public partial int this[ref readonly int i] { get => i; set { } }
                 Diagnostic(ErrorCode.ERR_PartialPropertyMissingDefinition, "this").WithArguments("C1.this[ref readonly int]").WithLocation(4, 24),
                 // (4,24): error CS0111: Type 'C1' already defines a member called 'this' with the same parameter types
                 //     public partial int this[ref readonly int i] { get => i; set { } }
                 Diagnostic(ErrorCode.ERR_MemberAlreadyExists, "this").WithArguments("this", "C1").WithLocation(4, 24),
-                // (9,24): error CS9300: Partial property 'C2.this[ref readonly int]' must have an implementation part.
+                // (9,24): error CS9248: Partial property 'C2.this[ref readonly int]' must have an implementation part.
                 //     public partial int this[ref readonly int i] { get; set; }
                 Diagnostic(ErrorCode.ERR_PartialPropertyMissingImplementation, "this").WithArguments("C2.this[ref readonly int]").WithLocation(9, 24),
-                // (10,24): error CS9301: Partial property 'C2.this[in int]' must have a definition part.
+                // (10,24): error CS9249: Partial property 'C2.this[in int]' must have a definition part.
                 //     public partial int this[in int i] { get => i; set { } }
                 Diagnostic(ErrorCode.ERR_PartialPropertyMissingDefinition, "this").WithArguments("C2.this[in int]").WithLocation(10, 24),
                 // (10,24): error CS0111: Type 'C2' already defines a member called 'this' with the same parameter types
@@ -2593,10 +2715,10 @@ partial class C2
 
             var comp = CreateCompilation(source);
             comp.VerifyEmitDiagnostics(
-                // (3,24): error CS9300: Partial property 'C1.this[in int]' must have an implementation part.
+                // (3,24): error CS9248: Partial property 'C1.this[in int]' must have an implementation part.
                 //     public partial int this[in int i] { get; set; }
                 Diagnostic(ErrorCode.ERR_PartialPropertyMissingImplementation, "this").WithArguments($"C1.this[{goodRefKind} int]").WithLocation(3, 24),
-                // (4,24): error CS9301: Partial property 'C1.this[ref int]' must have a definition part.
+                // (4,24): error CS9249: Partial property 'C1.this[ref int]' must have a definition part.
                 //     public partial int this[ref int i] { get => i; set { } }
                 Diagnostic(ErrorCode.ERR_PartialPropertyMissingDefinition, "this").WithArguments($"C1.this[{badRefKind} int]").WithLocation(4, 24),
                 // (4,24): error CS0111: Type 'C1' already defines a member called 'this' with the same parameter types
@@ -2605,13 +2727,13 @@ partial class C2
                 // (4,29): error CS0631: ref and out are not valid in this context
                 //     public partial int this[ref int i] { get => i; set { } }
                 Diagnostic(ErrorCode.ERR_IllegalRefParam, badRefKind).WithLocation(4, 29),
-                // (9,24): error CS9300: Partial property 'C2.this[ref int]' must have an implementation part.
+                // (9,24): error CS9248: Partial property 'C2.this[ref int]' must have an implementation part.
                 //     public partial int this[ref int i] { get; set; }
                 Diagnostic(ErrorCode.ERR_PartialPropertyMissingImplementation, "this").WithArguments($"C2.this[{badRefKind} int]").WithLocation(9, 24),
                 // (9,29): error CS0631: ref and out are not valid in this context
                 //     public partial int this[ref int i] { get; set; }
                 Diagnostic(ErrorCode.ERR_IllegalRefParam, badRefKind).WithLocation(9, 29),
-                // (10,24): error CS9301: Partial property 'C2.this[in int]' must have a definition part.
+                // (10,24): error CS9249: Partial property 'C2.this[in int]' must have a definition part.
                 //     public partial int this[in int i] { get => i; set { } }
                 Diagnostic(ErrorCode.ERR_PartialPropertyMissingDefinition, "this").WithArguments($"C2.this[{goodRefKind} int]").WithLocation(10, 24),
                 // (10,24): error CS0111: Type 'C2' already defines a member called 'this' with the same parameter types
@@ -2632,10 +2754,10 @@ partial class C
 
             var comp = CreateCompilation(source);
             comp.VerifyEmitDiagnostics(
-                // (3,24): error CS9300: Partial property 'C.this[int]' must have an implementation part.
+                // (3,24): error CS9248: Partial property 'C.this[int]' must have an implementation part.
                 //     public partial int this[int i] { get; set; }
                 Diagnostic(ErrorCode.ERR_PartialPropertyMissingImplementation, "this").WithArguments("C.this[int]").WithLocation(3, 24),
-                // (4,24): error CS9301: Partial property 'C.this[string]' must have a definition part.
+                // (4,24): error CS9249: Partial property 'C.this[string]' must have a definition part.
                 //     public partial int this[string s] { get => 1; set { } }
                 Diagnostic(ErrorCode.ERR_PartialPropertyMissingDefinition, "this").WithArguments("C.this[string]").WithLocation(4, 24));
         }
@@ -2654,7 +2776,7 @@ partial class C
 
             var comp = CreateCompilation(source);
             comp.VerifyEmitDiagnostics(
-                // (5,24): warning CS9308: Partial property declarations 'int C.this[string s]' and 'int C.this[string? s]' have signature differences.
+                // (5,24): warning CS9256: Partial property declarations 'int C.this[string s]' and 'int C.this[string? s]' have signature differences.
                 //     public partial int this[string? s] { get => 1; set { } }
                 Diagnostic(ErrorCode.WRN_PartialPropertySignatureDifference, "this").WithArguments("int C.this[string s]", "int C.this[string? s]").WithLocation(5, 24));
         }
@@ -2673,7 +2795,7 @@ partial class C
 
             var comp = CreateCompilation(source);
             comp.VerifyEmitDiagnostics(
-                // (5,24): warning CS9308: Partial property declarations 'int C.this[dynamic[] s]' and 'int C.this[object[] s]' have signature differences. 
+                // (5,24): warning CS9256: Partial property declarations 'int C.this[dynamic[] s]' and 'int C.this[object[] s]' have signature differences. 
                 //     public partial int this[object[] s] { get => 1; set { } }
                 Diagnostic(ErrorCode.WRN_PartialPropertySignatureDifference, "this").WithArguments("int C.this[dynamic[] s]", "int C.this[object[] s]").WithLocation(5, 24));
         }
@@ -3095,19 +3217,19 @@ partial class C
 
             var comp = CreateCompilation(source);
             comp.VerifyEmitDiagnostics(
-                // (3,24): error CS9300: Partial property 'C.this[int]' must have an implementation part.
+                // (3,24): error CS9248: Partial property 'C.this[int]' must have an implementation part.
                 //     public partial int this[int x] { get; set; } // missing impl
                 Diagnostic(ErrorCode.ERR_PartialPropertyMissingImplementation, "this").WithArguments("C.this[int]").WithLocation(3, 24),
-                // (5,24): error CS9301: Partial property 'C.this[int, int]' must have a definition part.
+                // (5,24): error CS9249: Partial property 'C.this[int, int]' must have a definition part.
                 //     public partial int this[int x, int y] { get => 1; set { } } // missing decl
                 Diagnostic(ErrorCode.ERR_PartialPropertyMissingDefinition, "this").WithArguments("C.this[int, int]").WithLocation(5, 24),
-                // (8,24): error CS9302: A partial property may not have multiple defining declarations, and cannot be an auto-property.
+                // (8,24): error CS9250: A partial property may not have multiple defining declarations, and cannot be an auto-property.
                 //     public partial int this[int x, int y, int z] { get; set; }
                 Diagnostic(ErrorCode.ERR_PartialPropertyDuplicateDefinition, "this").WithLocation(8, 24),
                 // (8,24): error CS0111: Type 'C' already defines a member called 'this' with the same parameter types
                 //     public partial int this[int x, int y, int z] { get; set; }
                 Diagnostic(ErrorCode.ERR_MemberAlreadyExists, "this").WithArguments("this", "C").WithLocation(8, 24),
-                // (13,24): error CS9303: A partial property may not have multiple implementing declarations
+                // (13,24): error CS9251: A partial property may not have multiple implementing declarations
                 //     public partial int this[int x, int y, int z, int a] { get => 1; set { } }
                 Diagnostic(ErrorCode.ERR_PartialPropertyDuplicateImplementation, "this").WithLocation(13, 24),
                 // (13,24): error CS0111: Type 'C' already defines a member called 'this' with the same parameter types
@@ -3128,7 +3250,7 @@ partial class C
 
             var comp = CreateCompilation(source);
             comp.VerifyEmitDiagnostics(
-                // (4,29): error CS9307: Both partial property declarations must have the same type.
+                // (4,29): error CS9255: Both partial property declarations must have the same type.
                 //     public partial string[] this[int x] { get => []; set { } }
                 Diagnostic(ErrorCode.ERR_PartialPropertyTypeDifference, "this").WithLocation(4, 29));
         }
@@ -3627,6 +3749,170 @@ partial class C
             AssertEx.Equal([], property.GetAttributes().ToStrings());
         }
 
+        [Fact]
+        public void PropertyInitializer()
+        {
+            var source = """
+                partial class C
+                {
+                    public partial string P1 { get; set; } = "a";
+                    public partial string P1 { get => ""; set { } }
+
+                    public partial string P2 { get; set; }
+                    public partial string P2 { get => ""; set { } } = "b";
+
+                    public partial string P3 { get; set; } = "c";
+                    public partial string P3 { get => ""; set { } } = "d";
+                }
+                """;
+
+            var comp = CreateCompilation(source);
+            comp.VerifyEmitDiagnostics(
+                // (3,27): error CS8050: Only auto-implemented properties can have initializers.
+                //     public partial string P1 { get; set; } = "a";
+                Diagnostic(ErrorCode.ERR_InitializerOnNonAutoProperty, "P1").WithLocation(3, 27),
+                // (7,27): error CS8050: Only auto-implemented properties can have initializers.
+                //     public partial string P2 { get => ""; set { } } = "b";
+                Diagnostic(ErrorCode.ERR_InitializerOnNonAutoProperty, "P2").WithLocation(7, 27),
+                // (9,27): error CS8050: Only auto-implemented properties can have initializers.
+                //     public partial string P3 { get; set; } = "c";
+                Diagnostic(ErrorCode.ERR_InitializerOnNonAutoProperty, "P3").WithLocation(9, 27),
+                // (10,27): error CS8050: Only auto-implemented properties can have initializers.
+                //     public partial string P3 { get => ""; set { } } = "d";
+                Diagnostic(ErrorCode.ERR_InitializerOnNonAutoProperty, "P3").WithLocation(10, 27));
+        }
+
+        [Fact]
+        public void PropertyInitializer_ContainsErrors()
+        {
+            var source = """
+                partial class C
+                {
+                    public partial string P1 { get; set; } = ERROR;
+                    public partial string P1 { get => ""; set { } }
+
+                    public partial string P2 { get; set; }
+                    public partial string P2 { get => ""; set { } } = ERROR;
+
+                    public partial string P3 { get; set; } = ERROR;
+                    public partial string P3 { get => ""; set { } } = ERROR;
+                }
+                """;
+
+            var comp = CreateCompilation(source);
+            comp.VerifyEmitDiagnostics(
+                // (3,27): error CS8050: Only auto-implemented properties can have initializers.
+                //     public partial string P1 { get; set; } = ERROR;
+                Diagnostic(ErrorCode.ERR_InitializerOnNonAutoProperty, "P1").WithLocation(3, 27),
+                // (3,46): error CS0103: The name 'ERROR' does not exist in the current context
+                //     public partial string P1 { get; set; } = ERROR;
+                Diagnostic(ErrorCode.ERR_NameNotInContext, "ERROR").WithArguments("ERROR").WithLocation(3, 46),
+                // (7,27): error CS8050: Only auto-implemented properties can have initializers.
+                //     public partial string P2 { get => ""; set { } } = ERROR;
+                Diagnostic(ErrorCode.ERR_InitializerOnNonAutoProperty, "P2").WithLocation(7, 27),
+                // (7,55): error CS0103: The name 'ERROR' does not exist in the current context
+                //     public partial string P2 { get => ""; set { } } = ERROR;
+                Diagnostic(ErrorCode.ERR_NameNotInContext, "ERROR").WithArguments("ERROR").WithLocation(7, 55),
+                // (9,27): error CS8050: Only auto-implemented properties can have initializers.
+                //     public partial string P3 { get; set; } = ERROR;
+                Diagnostic(ErrorCode.ERR_InitializerOnNonAutoProperty, "P3").WithLocation(9, 27),
+                // (9,46): error CS0103: The name 'ERROR' does not exist in the current context
+                //     public partial string P3 { get; set; } = ERROR;
+                Diagnostic(ErrorCode.ERR_NameNotInContext, "ERROR").WithArguments("ERROR").WithLocation(9, 46),
+                // (10,27): error CS8050: Only auto-implemented properties can have initializers.
+                //     public partial string P3 { get => ""; set { } } = ERROR;
+                Diagnostic(ErrorCode.ERR_InitializerOnNonAutoProperty, "P3").WithLocation(10, 27),
+                // (10,55): error CS0103: The name 'ERROR' does not exist in the current context
+                //     public partial string P3 { get => ""; set { } } = ERROR;
+                Diagnostic(ErrorCode.ERR_NameNotInContext, "ERROR").WithArguments("ERROR").WithLocation(10, 55));
+        }
+
+        [Fact]
+        public void PropertyInitializer_AndFieldTargetedAttribute()
+        {
+            // A synthesized field symbol is created when property has an initializer, even if the property is not an auto-property.
+            var source = """
+                public class Attr1 : System.Attribute { }
+                public class Attr2 : System.Attribute { }
+
+                partial class C
+                {
+                    [field: Attr1]
+                    public partial string P1 { get; set; } = "a";
+                    [field: Attr2]
+                    public partial string P1 { get => ""; set { } }
+
+                    [field: Attr1]
+                    public partial string P2 { get; set; }
+                    [field: Attr2]
+                    public partial string P2 { get => ""; set { } } = "b";
+
+                    [field: Attr1]
+                    public partial string P3 { get; set; } = "c";
+                    [field: Attr2]
+                    public partial string P3 { get => ""; set { } } = "d";
+                }
+                """;
+
+            var comp = CreateCompilation(source);
+            comp.VerifyEmitDiagnostics(
+                // (6,6): warning CS0657: 'field' is not a valid attribute location for this declaration. Valid attribute locations for this declaration are 'property'. All attributes in this block will be ignored.
+                //     [field: Attr1]
+                Diagnostic(ErrorCode.WRN_AttributeLocationOnBadDeclaration, "field").WithArguments("field", "property").WithLocation(6, 6),
+                // (7,27): error CS8050: Only auto-implemented properties can have initializers.
+                //     public partial string P1 { get; set; } = "a";
+                Diagnostic(ErrorCode.ERR_InitializerOnNonAutoProperty, "P1").WithLocation(7, 27),
+                // (8,6): warning CS0657: 'field' is not a valid attribute location for this declaration. Valid attribute locations for this declaration are 'property'. All attributes in this block will be ignored.
+                //     [field: Attr2]
+                Diagnostic(ErrorCode.WRN_AttributeLocationOnBadDeclaration, "field").WithArguments("field", "property").WithLocation(8, 6),
+                // (11,6): warning CS0657: 'field' is not a valid attribute location for this declaration. Valid attribute locations for this declaration are 'property'. All attributes in this block will be ignored.
+                //     [field: Attr1]
+                Diagnostic(ErrorCode.WRN_AttributeLocationOnBadDeclaration, "field").WithArguments("field", "property").WithLocation(11, 6),
+                // (13,6): warning CS0657: 'field' is not a valid attribute location for this declaration. Valid attribute locations for this declaration are 'property'. All attributes in this block will be ignored.
+                //     [field: Attr2]
+                Diagnostic(ErrorCode.WRN_AttributeLocationOnBadDeclaration, "field").WithArguments("field", "property").WithLocation(13, 6),
+                // (14,27): error CS8050: Only auto-implemented properties can have initializers.
+                //     public partial string P2 { get => ""; set { } } = "b";
+                Diagnostic(ErrorCode.ERR_InitializerOnNonAutoProperty, "P2").WithLocation(14, 27),
+                // (16,6): warning CS0657: 'field' is not a valid attribute location for this declaration. Valid attribute locations for this declaration are 'property'. All attributes in this block will be ignored.
+                //     [field: Attr1]
+                Diagnostic(ErrorCode.WRN_AttributeLocationOnBadDeclaration, "field").WithArguments("field", "property").WithLocation(16, 6),
+                // (17,27): error CS8050: Only auto-implemented properties can have initializers.
+                //     public partial string P3 { get; set; } = "c";
+                Diagnostic(ErrorCode.ERR_InitializerOnNonAutoProperty, "P3").WithLocation(17, 27),
+                // (18,6): warning CS0657: 'field' is not a valid attribute location for this declaration. Valid attribute locations for this declaration are 'property'. All attributes in this block will be ignored.
+                //     [field: Attr2]
+                Diagnostic(ErrorCode.WRN_AttributeLocationOnBadDeclaration, "field").WithArguments("field", "property").WithLocation(18, 6),
+                // (19,27): error CS8050: Only auto-implemented properties can have initializers.
+                //     public partial string P3 { get => ""; set { } } = "d";
+                Diagnostic(ErrorCode.ERR_InitializerOnNonAutoProperty, "P3").WithLocation(19, 27));
+
+            AssertEx.Equal([
+                "System.String C.<P1>k__BackingField",
+                "System.String C.P1 { get; set; }",
+                "System.String C.P1.get",
+                "void C.P1.set",
+                "System.String C.P2 { get; set; }",
+                "System.String C.P2.get",
+                "void C.P2.set",
+                "System.String C.<P2>k__BackingField",
+                "System.String C.<P3>k__BackingField",
+                "System.String C.P3 { get; set; }",
+                "System.String C.P3.get",
+                "void C.P3.set",
+                "System.String C.<P3>k__BackingField",
+                "C..ctor()"],
+                comp.GetMember<NamedTypeSymbol>("C").GetMembers().SelectAsArray(m => m.ToTestDisplayString()));
+
+            Assert.Empty(comp.GetMember<FieldSymbol>("C.<P1>k__BackingField").GetAttributes());
+            Assert.Empty(comp.GetMember<FieldSymbol>("C.<P2>k__BackingField").GetAttributes());
+
+            var p3Fields = comp.GetMembers("C.<P3>k__BackingField");
+            Assert.Equal(2, p3Fields.Length);
+            Assert.Empty(p3Fields[0].GetAttributes());
+            Assert.Empty(p3Fields[1].GetAttributes());
+        }
+
         [Theory]
         [InlineData("", "[UnscopedRef] ")]
         [InlineData("[UnscopedRef] ", "")]
@@ -4334,6 +4620,7 @@ partial class C
                 Diagnostic(ErrorCode.ERR_UnmanagedCallersOnlyRequiresStatic, "UnmanagedCallersOnly").WithLocation(17, 30));
         }
 
+        /// <remarks>See also <cref name="DocumentationCommentCompilerTests.PartialIndexer_Paramref_03"/></remarks>
         [Fact]
         public void IndexerParameterNameDifference()
         {
@@ -4343,7 +4630,7 @@ public void IndexerParameterNameDifference()
                 partial class C
                 {
                     public partial int this[int p1] { get; set; }
-                    public partial int this[int p2] { get => p2; set { } }
+                    public partial int this[int p2] { get => p2; set => p2.ToString(); }
 
                     static void Main()
                     {
@@ -4353,14 +4640,98 @@ static void Main()
                 }
                 """;
 
-            var verifier = CompileAndVerify(source, expectedOutput: "1");
+            var verifier = CompileAndVerify(source, expectedOutput: "1", symbolValidator: verify, sourceSymbolValidator: verify);
             verifier.VerifyDiagnostics(
-                // (6,24): warning CS9308: Partial property declarations 'int C.this[int p1]' and 'int C.this[int p2]' have signature differences.
+                // (6,24): warning CS9256: Partial property declarations 'int C.this[int p1]' and 'int C.this[int p2]' have signature differences.
                 //     public partial int this[int p2] { get => p2; set { } }
                 Diagnostic(ErrorCode.WRN_PartialPropertySignatureDifference, "this").WithArguments("int C.this[int p1]", "int C.this[int p2]").WithLocation(6, 24));
 
-            var indexer = ((CSharpCompilation)verifier.Compilation).GetMember<NamedTypeSymbol>("C").Indexers.Single();
-            Assert.Equal("p1", indexer.Parameters.Single().Name);
+            void verify(ModuleSymbol module)
+            {
+                var indexer = module.GlobalNamespace.GetMember<NamedTypeSymbol>("C").Indexers.Single();
+                Assert.Equal("p1", indexer.Parameters.Single().Name);
+            }
+        }
+
+        /// <remarks>See also <cref name="DocumentationCommentCompilerTests.PartialIndexer_Paramref_03"/></remarks>
+        [Fact]
+        public void IndexerParameterNameDifference_Attributes()
+        {
+            var source = """
+                using System;
+
+                [AttributeUsage(AttributeTargets.All, AllowMultiple = true)]
+                public class Attr(string s) : Attribute { } // 1 (unread parameter)
+
+                partial class C
+                {
+                    [Attr(nameof(p1))]
+                    [Attr(nameof(p2))] // 2
+                    public partial int this[int p1]
+                    {
+                        get;
+
+                        [param: Attr(nameof(p1))] // 3
+                        [param: Attr(nameof(p2))]
+                        set;
+                    }
+
+                    [Attr(nameof(p1))]
+                    [Attr(nameof(p2))] // 4
+                    public partial int this[int p2] // 5
+                    {
+                        get => p2;
+
+                        [param: Attr(nameof(p1))] // 6
+                        [param: Attr(nameof(p2))]
+                        set => p2.ToString();
+                    }
+
+                    [Attr(nameof(p1))]
+                    [Attr(nameof(p2))] // 7
+                    public partial void M(
+                        [Attr(nameof(p1))] // 8
+                        [Attr(nameof(p2))]
+                        int p1);
+                    public partial void M( // 9
+                        [Attr(nameof(p1))] // 10
+                        [Attr(nameof(p2))]
+                        int p2) { }
+                }
+                """;
+
+            var verifier = CreateCompilation(source);
+            verifier.VerifyDiagnostics(
+                // (4,26): warning CS9113: Parameter 's' is unread.
+                // public class Attr(string s) : Attribute { } // 1 (unread parameter)
+                Diagnostic(ErrorCode.WRN_UnreadPrimaryConstructorParameter, "s").WithArguments("s").WithLocation(4, 26),
+                // (9,18): error CS0103: The name 'p2' does not exist in the current context
+                //     [Attr(nameof(p2))] // 2
+                Diagnostic(ErrorCode.ERR_NameNotInContext, "p2").WithArguments("p2").WithLocation(9, 18),
+                // (14,29): error CS0103: The name 'p1' does not exist in the current context
+                //         [param: Attr(nameof(p1))] // 3
+                Diagnostic(ErrorCode.ERR_NameNotInContext, "p1").WithArguments("p1").WithLocation(14, 29),
+                // (20,18): error CS0103: The name 'p2' does not exist in the current context
+                //     [Attr(nameof(p2))] // 4
+                Diagnostic(ErrorCode.ERR_NameNotInContext, "p2").WithArguments("p2").WithLocation(20, 18),
+                // (21,24): warning CS9256: Partial property declarations 'int C.this[int p1]' and 'int C.this[int p2]' have signature differences.
+                //     public partial int this[int p2] // 5
+                Diagnostic(ErrorCode.WRN_PartialPropertySignatureDifference, "this").WithArguments("int C.this[int p1]", "int C.this[int p2]").WithLocation(21, 24),
+                // (25,29): error CS0103: The name 'p1' does not exist in the current context
+                //         [param: Attr(nameof(p1))] // 6
+                Diagnostic(ErrorCode.ERR_NameNotInContext, "p1").WithArguments("p1").WithLocation(25, 29),
+                // (31,18): error CS0103: The name 'p2' does not exist in the current context
+                //     [Attr(nameof(p2))] // 7
+                Diagnostic(ErrorCode.ERR_NameNotInContext, "p2").WithArguments("p2").WithLocation(31, 18),
+                // (33,22): error CS0103: The name 'p1' does not exist in the current context
+                //         [Attr(nameof(p1))] // 8
+                Diagnostic(ErrorCode.ERR_NameNotInContext, "p1").WithArguments("p1").WithLocation(33, 22),
+                // (36,25): warning CS8826: Partial method declarations 'void C.M(int p1)' and 'void C.M(int p2)' have signature differences.
+                //     public partial void M( // 9
+                Diagnostic(ErrorCode.WRN_PartialMethodTypeDifference, "M").WithArguments("void C.M(int p1)", "void C.M(int p2)").WithLocation(36, 25),
+                // (37,22): error CS0103: The name 'p1' does not exist in the current context
+                //         [Attr(nameof(p1))] // 10
+                Diagnostic(ErrorCode.ERR_NameNotInContext, "p1").WithArguments("p1").WithLocation(37, 22));
         }
 
         [Fact]
@@ -4602,7 +4973,5 @@ partial class C
             Assert.Equal("SourceFile(Program.cs[52..53))", defSymbol.Locations.Single().ToString());
             Assert.Equal("SourceFile(Program.cs[97..98))", implSymbol.Locations.Single().ToString());
         }
-
-        // PROTOTYPE(partial-properties): override partial property where base has modopt
     }
 }
diff --git a/src/Compilers/CSharp/Test/Symbol/Symbols/Source/CustomModifierCopyTests.cs b/src/Compilers/CSharp/Test/Symbol/Symbols/Source/CustomModifierCopyTests.cs
index b1068ddbae44f..27b140fe6ef97 100644
--- a/src/Compilers/CSharp/Test/Symbol/Symbols/Source/CustomModifierCopyTests.cs
+++ b/src/Compilers/CSharp/Test/Symbol/Symbols/Source/CustomModifierCopyTests.cs
@@ -269,6 +269,71 @@ class Derived : MethodCustomModifierCombinations
             }
         }
 
+        /// <summary>
+        /// Test copying custom modifiers in/on parameters/return types.
+        /// </summary>
+        [Fact]
+        public void TestPartialMethodOverrideCombinations()
+        {
+            var text = @"
+partial class Derived : MethodCustomModifierCombinations
+{
+    public override partial int[] Method1111(int[] a) { return a; }
+    public override partial int[] Method1110(int[] a) { return a; }
+    public override partial int[] Method1101(int[] a) { return a; }
+    public override partial int[] Method1100(int[] a) { return a; }
+    public override partial int[] Method1011(int[] a) { return a; }
+    public override partial int[] Method1010(int[] a) { return a; }
+    public override partial int[] Method1001(int[] a) { return a; }
+    public override partial int[] Method1000(int[] a) { return a; }
+    public override partial int[] Method0111(int[] a) { return a; }
+    public override partial int[] Method0110(int[] a) { return a; }
+    public override partial int[] Method0101(int[] a) { return a; }
+    public override partial int[] Method0100(int[] a) { return a; }
+    public override partial int[] Method0011(int[] a) { return a; }
+    public override partial int[] Method0010(int[] a) { return a; }
+    public override partial int[] Method0001(int[] a) { return a; }
+    public override partial int[] Method0000(int[] a) { return a; }
+
+    public override partial int[] Method1111(int[] a);
+    public override partial int[] Method1110(int[] a);
+    public override partial int[] Method1101(int[] a);
+    public override partial int[] Method1100(int[] a);
+    public override partial int[] Method1011(int[] a);
+    public override partial int[] Method1010(int[] a);
+    public override partial int[] Method1001(int[] a);
+    public override partial int[] Method1000(int[] a);
+    public override partial int[] Method0111(int[] a);
+    public override partial int[] Method0110(int[] a);
+    public override partial int[] Method0101(int[] a);
+    public override partial int[] Method0100(int[] a);
+    public override partial int[] Method0011(int[] a);
+    public override partial int[] Method0010(int[] a);
+    public override partial int[] Method0001(int[] a);
+    public override partial int[] Method0000(int[] a);
+}
+";
+
+            var ilAssemblyReference = TestReferences.SymbolsTests.CustomModifiers.Modifiers.dll;
+
+            var comp = CreateCompilation(text, new MetadataReference[] { ilAssemblyReference });
+            var global = comp.GlobalNamespace;
+
+            comp.VerifyEmitDiagnostics();
+
+            var @class = global.GetMember<NamedTypeSymbol>("Derived");
+
+            for (int i = 0; i < 0xf; i++)
+            {
+                CheckMethodCustomModifiers(
+                    @class.GetMember<MethodSymbol>("Method" + Convert.ToString(i, 2).PadLeft(4, '0')),
+                    inReturnType: (i & 0x8) != 0,
+                    onReturnType: (i & 0x4) != 0,
+                    inParameterType: (i & 0x2) != 0,
+                    onParameterType: (i & 0x1) != 0);
+            }
+        }
+
         /// <summary>
         /// Test copying custom modifiers in/on property types.
         /// </summary>
@@ -290,7 +355,61 @@ class Derived : PropertyCustomModifierCombinations
             var comp = CreateCompilation(text, new MetadataReference[] { ilAssemblyReference });
             var global = comp.GlobalNamespace;
 
-            comp.VerifyDiagnostics();
+            comp.VerifyEmitDiagnostics();
+
+            var @class = global.GetMember<NamedTypeSymbol>("Derived");
+
+            for (int i = 0; i < 0x4; i++)
+            {
+                PropertySymbol property = @class.GetMember<PropertySymbol>("Property" + Convert.ToString(i, 2).PadLeft(2, '0'));
+                bool inType = (i & 0x2) != 0;
+                bool onType = (i & 0x1) != 0;
+
+                CheckPropertyCustomModifiers(property, inType, onType);
+                CheckMethodCustomModifiers(
+                    property.GetMethod,
+                    inReturnType: inType,
+                    onReturnType: onType,
+                    inParameterType: false,
+                    onParameterType: false);
+                CheckMethodCustomModifiers(
+                    property.SetMethod,
+                    inReturnType: false,
+                    onReturnType: false,
+                    inParameterType: inType,
+                    onParameterType: onType);
+            }
+        }
+
+        /// <summary>
+        /// Test copying custom modifiers in/on partial property types.
+        /// </summary>
+        [Fact]
+        public void TestPartialPropertyOverrideCombinations()
+        {
+            var text = @"
+partial class Derived : PropertyCustomModifierCombinations
+{
+    public override partial int[] Property11 { get; set; }
+    public override partial int[] Property11 { get => new int[0]; set { } }
+
+    public override partial int[] Property10 { get; set; }
+    public override partial int[] Property10 { get => new int[0]; set { } }
+
+    public override partial int[] Property01 { get; set; }
+    public override partial int[] Property01 { get => new int[0]; set { } }
+
+    public override partial int[] Property00 { get; set; }
+    public override partial int[] Property00 { get => new int[0]; set { } }
+}
+";
+
+            var ilAssemblyReference = TestReferences.SymbolsTests.CustomModifiers.Modifiers.dll;
+
+            var comp = CreateCompilation(text, new MetadataReference[] { ilAssemblyReference });
+            var global = comp.GlobalNamespace;
+
+            comp.VerifyEmitDiagnostics();
 
             var @class = global.GetMember<NamedTypeSymbol>("Derived");
 
diff --git a/src/Compilers/CSharp/Test/Symbol/Symbols/SymbolErrorTests.cs b/src/Compilers/CSharp/Test/Symbol/Symbols/SymbolErrorTests.cs
index f4203a5ce3cab..97aca352abf83 100644
--- a/src/Compilers/CSharp/Test/Symbol/Symbols/SymbolErrorTests.cs
+++ b/src/Compilers/CSharp/Test/Symbol/Symbols/SymbolErrorTests.cs
@@ -14087,10 +14087,10 @@ partial int this[int index]
                 // (4,5): error CS0267: The 'partial' modifier can only appear immediately before 'class', 'record', 'struct', 'interface', or a method or property return type.
                 //     partial int f;
                 Diagnostic(ErrorCode.ERR_PartialMisplaced, "partial").WithLocation(4, 5),
-                // (5,20): error CS9301: Partial property 'C.P' must have a definition part.
+                // (5,20): error CS9249: Partial property 'C.P' must have a definition part.
                 //     partial object P { get { return null; } }
                 Diagnostic(ErrorCode.ERR_PartialPropertyMissingDefinition, "P").WithArguments("C.P").WithLocation(5, 20),
-                // (6,17): error CS9301: Partial property 'C.this[int]' must have a definition part.
+                // (6,17): error CS9249: Partial property 'C.this[int]' must have a definition part.
                 //     partial int this[int index]
                 Diagnostic(ErrorCode.ERR_PartialPropertyMissingDefinition, "this").WithArguments("C.this[int]").WithLocation(6, 17),
                 // (4,17): warning CS0169: The field 'C.f' is never used
diff --git a/src/Compilers/Core/Portable/DiagnosticAnalyzer/AnalyzerManager.AnalyzerExecutionContext.cs b/src/Compilers/Core/Portable/DiagnosticAnalyzer/AnalyzerManager.AnalyzerExecutionContext.cs
index db54971ea9269..40acf22276ee5 100644
--- a/src/Compilers/Core/Portable/DiagnosticAnalyzer/AnalyzerManager.AnalyzerExecutionContext.cs
+++ b/src/Compilers/Core/Portable/DiagnosticAnalyzer/AnalyzerManager.AnalyzerExecutionContext.cs
@@ -204,6 +204,7 @@ void processMembers(IEnumerable<ISymbol> members)
                                 memberSet.Add(member);
 
                                 // Ensure that we include symbols for both parts of partial methods.
+                                // https://github.com/dotnet/roslyn/issues/73772: also cascade to partial property implementation part
                                 if (member is IMethodSymbol method &&
                                     !(method.PartialImplementationPart is null))
                                 {
diff --git a/src/Compilers/Core/Portable/DiagnosticAnalyzer/CompilationWithAnalyzers.cs b/src/Compilers/Core/Portable/DiagnosticAnalyzer/CompilationWithAnalyzers.cs
index 2f8d1fa1fd9f7..a23c7ef140d3f 100644
--- a/src/Compilers/Core/Portable/DiagnosticAnalyzer/CompilationWithAnalyzers.cs
+++ b/src/Compilers/Core/Portable/DiagnosticAnalyzer/CompilationWithAnalyzers.cs
@@ -1118,6 +1118,10 @@ static bool shouldIncludeSymbol(ISymbolInternal symbol, SyntaxTree tree, Cancell
                         }
                     }
 
+                    // https://github.com/dotnet/roslyn/issues/73772: should we also check IPropertySymbol?
+                    // there is no interface IPropertySymbolInternal
+                    // where are tests for this?
+
                     return false;
                 }
             }
diff --git a/src/Compilers/Core/Portable/Emit/EditAndContinue/SymbolChanges.cs b/src/Compilers/Core/Portable/Emit/EditAndContinue/SymbolChanges.cs
index 88c563a21e702..89c2cf1fd101a 100644
--- a/src/Compilers/Core/Portable/Emit/EditAndContinue/SymbolChanges.cs
+++ b/src/Compilers/Core/Portable/Emit/EditAndContinue/SymbolChanges.cs
@@ -414,6 +414,7 @@ private void CalculateChanges(
 
                 // Partial methods are supplied as implementations but recorded
                 // internally as definitions since definitions are used in emit.
+                // https://github.com/dotnet/roslyn/issues/73772: should we also make sure to use the definition for a partial property?
                 if (newMember.Kind == SymbolKind.Method)
                 {
                     var newMethod = (IMethodSymbolInternal)newMember;
diff --git a/src/Compilers/Core/Portable/Emit/SemanticEdit.cs b/src/Compilers/Core/Portable/Emit/SemanticEdit.cs
index be2f00be20c11..14cec150ebc50 100644
--- a/src/Compilers/Core/Portable/Emit/SemanticEdit.cs
+++ b/src/Compilers/Core/Portable/Emit/SemanticEdit.cs
@@ -142,6 +142,7 @@ public SemanticEdit(SemanticEditKind kind, ISymbol? oldSymbol, ISymbol? newSymbo
                 }
             }
 
+            // https://github.com/dotnet/roslyn/issues/73772: should we also do this check for partial properties?
             if (oldSymbol is IMethodSymbol { PartialImplementationPart: not null })
             {
                 throw new ArgumentException("Partial method implementation required", nameof(oldSymbol));
diff --git a/src/Compilers/Test/Core/Assert/AssertEx.cs b/src/Compilers/Test/Core/Assert/AssertEx.cs
index db5502c63f798..244b92a4d6aa9 100644
--- a/src/Compilers/Test/Core/Assert/AssertEx.cs
+++ b/src/Compilers/Test/Core/Assert/AssertEx.cs
@@ -656,13 +656,7 @@ public static string GetAssertMessage<T>(
                 }
                 else
                 {
-                    itemInspector = static obj => obj switch
-                    {
-                        null => "<null>",
-                        // PROTOTYPE(partial-properties): this is adding unwanted quotes to VerifyIL baselines
-                        string s => $@"""{s}""",
-                        _ => obj.ToString()
-                    };
+                    itemInspector = new Func<T, string>(obj => (obj != null) ? obj.ToString() : "<null>");
                 }
             }
 
diff --git a/src/EditorFeatures/CSharpTest/SymbolKey/SymbolKeyCompilationsTests.cs b/src/EditorFeatures/CSharpTest/SymbolKey/SymbolKeyCompilationsTests.cs
index 51430c5d8c820..b9f1ec2e3ae6c 100644
--- a/src/EditorFeatures/CSharpTest/SymbolKey/SymbolKeyCompilationsTests.cs
+++ b/src/EditorFeatures/CSharpTest/SymbolKey/SymbolKeyCompilationsTests.cs
@@ -160,6 +160,7 @@ public partial void M() { }
         var implementation = definition.PartialImplementationPart;
 
         // Assert that both the definition and implementation resolve back to themselves
+        // https://github.com/dotnet/roslyn/issues/73772: add a similar test for properties
         Assert.Equal(definition, ResolveSymbol(definition, comp, SymbolKeyComparison.None));
         Assert.Equal(implementation, ResolveSymbol(implementation, comp, SymbolKeyComparison.None));
     }
diff --git a/src/Features/Core/Portable/Completion/Providers/AbstractPartialMethodCompletionProvider.cs b/src/Features/Core/Portable/Completion/Providers/AbstractPartialMethodCompletionProvider.cs
index 11ef0079da66c..ddf3d0b4c1397 100644
--- a/src/Features/Core/Portable/Completion/Providers/AbstractPartialMethodCompletionProvider.cs
+++ b/src/Features/Core/Portable/Completion/Providers/AbstractPartialMethodCompletionProvider.cs
@@ -90,6 +90,7 @@ protected override async Task<ISymbol> GenerateMemberAsync(ISymbol member, IName
         if (enclosingSymbol.TypeKind is not (TypeKind.Struct or TypeKind.Class))
             return null;
 
+        // https://github.com/dotnet/roslyn/issues/73772: should we also add an `AbstractPartialPropertiesProvider`?
         var symbols = semanticModel.LookupSymbols(position, container: enclosingSymbol)
                                     .OfType<IMethodSymbol>()
                                     .Where(m => IsPartial(m) && m.PartialImplementationPart == null);
diff --git a/src/Features/Core/Portable/EditAndContinue/AbstractEditAndContinueAnalyzer.cs b/src/Features/Core/Portable/EditAndContinue/AbstractEditAndContinueAnalyzer.cs
index e8d2fd1d3de3c..769cf5bc507d0 100644
--- a/src/Features/Core/Portable/EditAndContinue/AbstractEditAndContinueAnalyzer.cs
+++ b/src/Features/Core/Portable/EditAndContinue/AbstractEditAndContinueAnalyzer.cs
@@ -3185,6 +3185,7 @@ IFieldSymbol or
                     // The partial type needs to be specified in the following cases:
                     // 1) partial method is updated (in case both implementation and definition are updated)
                     // 2) partial type is updated
+                    // https://github.com/dotnet/roslyn/issues/73772: do we also need to check IPropertySymbol.PartialDefinitionPart here?
                     var partialType = editKind == SemanticEditKind.Update && symbol is IMethodSymbol { PartialDefinitionPart: not null }
                         ? symbolCache.GetKey(symbol.ContainingType, cancellationToken)
                         : IsPartialTypeEdit(oldSymbol, newSymbol, oldTree, newTree)
@@ -3354,6 +3355,7 @@ bool PreprocessSymbolEdit(ref ISymbol? oldSymbol, ref ISymbol? newSymbol)
                     var result = symbolKey.Resolve(compilation, ignoreAssemblyKey: true, cancellationToken).Symbol;
 
                     // If we were looking for a definition and an implementation is returned the definition does not exist.
+                    // https://github.com/dotnet/roslyn/issues/73772: Does PartialDefinitionPart also need to be checked here?
                     return symbol is IMethodSymbol { PartialDefinitionPart: not null } && result is IMethodSymbol { IsPartialDefinition: true } ? null : result;
                 }
 
@@ -3666,6 +3668,7 @@ void AddDelete(ISymbol? symbol)
             if (symbol is null)
                 return;
 
+            // https://github.com/dotnet/roslyn/issues/73772
             Debug.Assert(symbol is not IMethodSymbol { IsPartialDefinition: true });
 
             var partialType = symbol is IMethodSymbol { PartialDefinitionPart: not null } ? SymbolKey.Create(symbol.ContainingType, cancellationToken) : (SymbolKey?)null;
diff --git a/src/Features/Core/Portable/EditAndContinue/Utilities/Extensions.cs b/src/Features/Core/Portable/EditAndContinue/Utilities/Extensions.cs
index 3123b0a18407f..0d2226145bc2a 100644
--- a/src/Features/Core/Portable/EditAndContinue/Utilities/Extensions.cs
+++ b/src/Features/Core/Portable/EditAndContinue/Utilities/Extensions.cs
@@ -183,6 +183,7 @@ private static bool ParsePrimaryParameterBackingFieldName(string fieldName, [Not
         => (IMethodSymbol?)constructor.ContainingType.GetMembers(WellKnownMemberNames.DeconstructMethodName).FirstOrDefault(
             static (symbol, constructor) => symbol is IMethodSymbol method && HasDeconstructorSignature(method, constructor), constructor)?.PartialAsImplementation();
 
+    // https://github.com/dotnet/roslyn/issues/73772: does this helper need to be updated to use IPropertySymbol.PartialImplementationPart?
     public static ISymbol PartialAsImplementation(this ISymbol symbol)
         => symbol is IMethodSymbol { PartialImplementationPart: { } impl } ? impl : symbol;
 
diff --git a/src/Features/TestUtilities/EditAndContinue/EditAndContinueTestVerifier.cs b/src/Features/TestUtilities/EditAndContinue/EditAndContinueTestVerifier.cs
index b4898d7604f13..cf39e98564710 100644
--- a/src/Features/TestUtilities/EditAndContinue/EditAndContinueTestVerifier.cs
+++ b/src/Features/TestUtilities/EditAndContinue/EditAndContinueTestVerifier.cs
@@ -322,6 +322,7 @@ SymbolKey CreateSymbolKey(SemanticEditDescription edit)
 
                         // Symbol key will happily resolve to a definition part that has no implementation, so we validate that
                         // differently
+                        // https://github.com/dotnet/roslyn/issues/73772: what about deletion of partial property?
                         if (expectedOldSymbol is IMethodSymbol { IsPartialDefinition: true } &&
                             symbolKey.Resolve(oldCompilation, ignoreAssemblyKey: true).Symbol is IMethodSymbol resolvedMethod)
                         {
diff --git a/src/Workspaces/Core/Portable/FindSymbols/FindReferences/Finders/OrdinaryMethodReferenceFinder.cs b/src/Workspaces/Core/Portable/FindSymbols/FindReferences/Finders/OrdinaryMethodReferenceFinder.cs
index a046043129790..6813b72dd61ba 100644
--- a/src/Workspaces/Core/Portable/FindSymbols/FindReferences/Finders/OrdinaryMethodReferenceFinder.cs
+++ b/src/Workspaces/Core/Portable/FindSymbols/FindReferences/Finders/OrdinaryMethodReferenceFinder.cs
@@ -34,6 +34,7 @@ protected override ValueTask<ImmutableArray<ISymbol>> DetermineCascadedSymbolsAs
 
     private static ImmutableArray<ISymbol> GetOtherPartsOfPartial(IMethodSymbol symbol)
     {
+        // https://github.com/dotnet/roslyn/issues/73772: define/use a similar helper for PropertySymbolReferenceFinder+PropertyAccessorSymbolReferenceFinder?
         if (symbol.PartialDefinitionPart != null)
             return [symbol.PartialDefinitionPart];
 
diff --git a/src/Workspaces/Core/Portable/FindSymbols/FindReferences/Finders/ParameterSymbolReferenceFinder.cs b/src/Workspaces/Core/Portable/FindSymbols/FindReferences/Finders/ParameterSymbolReferenceFinder.cs
index 5cffb6d4ca570..93978ac446a97 100644
--- a/src/Workspaces/Core/Portable/FindSymbols/FindReferences/Finders/ParameterSymbolReferenceFinder.cs
+++ b/src/Workspaces/Core/Portable/FindSymbols/FindReferences/Finders/ParameterSymbolReferenceFinder.cs
@@ -241,6 +241,7 @@ private static void CascadeBetweenPartialMethodParameters(
         IParameterSymbol parameter,
         ArrayBuilder<ISymbol> results)
     {
+        // https://github.com/dotnet/roslyn/issues/73772: also cascade partial indexer parameters
         if (parameter.ContainingSymbol is IMethodSymbol method)
         {
             var ordinal = parameter.Ordinal;
diff --git a/src/Workspaces/Core/Portable/Rename/ConflictEngine/DeclarationConflictHelpers.cs b/src/Workspaces/Core/Portable/Rename/ConflictEngine/DeclarationConflictHelpers.cs
index f683b96b5fe64..7e17a0ca24a75 100644
--- a/src/Workspaces/Core/Portable/Rename/ConflictEngine/DeclarationConflictHelpers.cs
+++ b/src/Workspaces/Core/Portable/Rename/ConflictEngine/DeclarationConflictHelpers.cs
@@ -55,6 +55,7 @@ private static ImmutableArray<Location> GetConflictLocations(ISymbol renamedMemb
         {
             if (signatureToConflictingMember.TryGetValue(signature, out var conflictingSymbol))
             {
+                // https://github.com/dotnet/roslyn/issues/73772: add other partial property part as conflicting symbol
                 if (isMethod && conflictingSymbol is IMethodSymbol conflictingMethod && renamedMember is IMethodSymbol renamedMethod)
                 {
                     if (!(conflictingMethod.PartialDefinitionPart != null && Equals(conflictingMethod.PartialDefinitionPart, renamedMethod)) &&