Address PR feedback.

src/Compilers/CSharp/Portable/Binder/Binder_Expressions.cs

@@ -4986,13 +4986,15 @@ private static void CheckRequiredMembersInObjectInitializer(
                 return;
             }
 
-            var requiredMembers = constructor.ContainingType.AllRequiredMembers.ToBuilder();
+            var requiredMembers = constructor.ContainingType.AllRequiredMembers;
 
             if (requiredMembers.Count == 0)
             {
                 return;
             }
 
+            var requiredMembersBuilder = requiredMembers.ToBuilder();
+
             if (initializers.IsDefaultOrEmpty)
             {
                 reportMembers();
@@ -5001,24 +5003,28 @@ private static void CheckRequiredMembersInObjectInitializer(
 
             foreach (var initializer in initializers)
             {
-                var (memberSymbol, initializerExpression) = initializer is BoundAssignmentOperator assignmentOperator
-                    ? (assignmentOperator.Left switch
-                    {
-                        // Regular initializers
-                        BoundObjectInitializerMember member => member.MemberSymbol,
-                        // Attribute initializers
-                        BoundPropertyAccess propertyAccess => propertyAccess.PropertySymbol,
-                        BoundFieldAccess fieldAccess => fieldAccess.FieldSymbol,
-                        _ => null
-                    }, assignmentOperator.Right)
-                    : default;
+                if (initializer is not BoundAssignmentOperator assignmentOperator)
+                {
+                    continue;
+                }
+
+                var memberSymbol = assignmentOperator.Left switch
+                {
+                    // Regular initializers
+                    BoundObjectInitializerMember member => member.MemberSymbol,
+                    // Attribute initializers
+                    BoundPropertyAccess propertyAccess => propertyAccess.PropertySymbol,
+                    BoundFieldAccess fieldAccess => fieldAccess.FieldSymbol,
+                    // Error cases
+                    _ => null
+                };
 
                 if (memberSymbol is null)
                 {
                     continue;
                 }
 
-                if (!requiredMembers.TryGetValue(memberSymbol.Name, out var requiredMember))
+                if (!requiredMembersBuilder.TryGetValue(memberSymbol.Name, out var requiredMember))
                 {
                     continue;
                 }
@@ -5028,12 +5034,12 @@ private static void CheckRequiredMembersInObjectInitializer(
                     continue;
                 }
 
-                requiredMembers.Remove(memberSymbol.Name);
+                requiredMembersBuilder.Remove(memberSymbol.Name);
 
-                if (initializerExpression is BoundObjectInitializerExpressionBase)
+                if (assignmentOperator.Right is BoundObjectInitializerExpressionBase initializerExpression)
                 {
                     // Required member '{0}' must be assigned a value, it cannot use a nested member or collection initializer.
-                    diagnostics.Add(ErrorCode.ERR_RequiredMembersMustBeAssignment, initializerExpression.Syntax.Location, requiredMember);
+                    diagnostics.Add(ErrorCode.ERR_RequiredMembersMustBeAssignedValue, initializerExpression.Syntax.Location, requiredMember);
                 }
             }
 
@@ -5049,7 +5055,7 @@ void reportMembers()
                     _ => creationSyntax.Location
                 };
 
-                foreach (var (_, member) in requiredMembers)
+                foreach (var (_, member) in requiredMembersBuilder)
                 {
                     // Required member '{0}' must be set in the object initializer or attribute constructor.
                     diagnostics.Add(ErrorCode.ERR_RequiredMemberMustBeSet, location, member);
@@ -5847,6 +5853,8 @@ internal bool TryPerformConstructorOverloadResolution(
                 diagnostics.AddDependencies(useSiteInfo);
                 foreach (var diagnostic in useSiteInfo.Diagnostics)
                 {
+                    // We don't want to report this error here because we'll report ERR_RequiredMembersBaseTypeInvalid. That error is suppressable by the
+                    // user using the `SetsRequiredMembers` attribute on the constructor, so reporting this error would prevent that from working.
                     if ((ErrorCode)diagnostic.Code == ErrorCode.ERR_RequiredMembersInvalid)
                     {
                         continue;

src/Compilers/CSharp/Portable/CSharpResources.resx

@@ -7010,7 +7010,7 @@ To remove the warning, you can use /reference instead (set the Embed Interop Typ
   <data name="ERR_RequiredMemberMustBeSet" xml:space="preserve">
     <value>Required member '{0}' must be set in the object initializer or attribute constructor.</value>
   </data>
-  <data name="ERR_RequiredMembersMustBeAssignment" xml:space="preserve">
+  <data name="ERR_RequiredMembersMustBeAssignedValue" xml:space="preserve">
     <value>Required member '{0}' must be assigned a value, it cannot use a nested member or collection initializer.</value>
   </data>
   <data name="ERR_RequiredMembersInvalid" xml:space="preserve">

src/Compilers/CSharp/Portable/Errors/ErrorCode.cs

@@ -2059,7 +2059,7 @@ internal enum ErrorCode
         ERR_ExplicitRequiredMember = 9504,
         ERR_RequiredMemberMustBeSettable = 9505,
         ERR_RequiredMemberMustBeSet = 9506,
-        ERR_RequiredMembersMustBeAssignment = 9507,
+        ERR_RequiredMembersMustBeAssignedValue = 9507,
         ERR_RequiredMembersInvalid = 9508,
         ERR_RequiredMembersBaseTypeInvalid = 9509,
     }

src/Compilers/CSharp/Test/Symbol/Symbols/RequiredMembersTests.cs

@@ -1388,13 +1388,33 @@ public class D
         comp.VerifyDiagnostics(
             // (2,24): error CS9507: Required member 'C.D1' must be assigned a value, it cannot use a nested member or collection initializer.
             // var c = new C() { D1 = { NestedProp = 1 }, D2 = { NestedProp = 2 } };
-            Diagnostic(ErrorCode.ERR_RequiredMembersMustBeAssignment, "{ NestedProp = 1 }").WithArguments("C.D1").WithLocation(2, 24),
+            Diagnostic(ErrorCode.ERR_RequiredMembersMustBeAssignedValue, "{ NestedProp = 1 }").WithArguments("C.D1").WithLocation(2, 24),
             // (2,49): error CS9507: Required member 'C.D2' must be assigned a value, it cannot use a nested member or collection initializer.
             // var c = new C() { D1 = { NestedProp = 1 }, D2 = { NestedProp = 2 } };
-            Diagnostic(ErrorCode.ERR_RequiredMembersMustBeAssignment, "{ NestedProp = 2 }").WithArguments("C.D2").WithLocation(2, 49)
+            Diagnostic(ErrorCode.ERR_RequiredMembersMustBeAssignedValue, "{ NestedProp = 2 }").WithArguments("C.D2").WithLocation(2, 49)
         );
     }
 
+    [Fact]
+    public void EnforcedRequiredMembers_NoInheritance_NestedObjectCreationAllowed()
+    {
+        var c = @"
+var c = new C() { D1 = new() { NestedProp = 1 }, D2 = new() { NestedProp = 2 } };
+
+public class C
+{
+    public required D D1 { get; set; }
+    public required D D2;
+}
+public class D
+{
+    public int NestedProp { get; set; }
+}
+";
+        var comp = CreateCompilationWithRequiredMembers(c);
+        CompileAndVerify(comp).VerifyDiagnostics();
+    }
+
     [Fact]
     public void EnforcedRequiredMembers_NoInheritance_DisallowedNestedCollectionInitializer()
     {
@@ -1412,13 +1432,30 @@ public class C
         comp.VerifyDiagnostics(
             // (3,24): error CS9507: Required member 'C.L1' must be assigned a value, it cannot use a nested member or collection initializer.
             // var c = new C() { L1 = { 1, 2, 3 }, L2 = { 4, 5, 6 } };
-            Diagnostic(ErrorCode.ERR_RequiredMembersMustBeAssignment, "{ 1, 2, 3 }").WithArguments("C.L1").WithLocation(3, 24),
+            Diagnostic(ErrorCode.ERR_RequiredMembersMustBeAssignedValue, "{ 1, 2, 3 }").WithArguments("C.L1").WithLocation(3, 24),
             // (3,42): error CS9507: Required member 'C.L2' must be assigned a value, it cannot use a nested member or collection initializer.
             // var c = new C() { L1 = { 1, 2, 3 }, L2 = { 4, 5, 6 } };
-            Diagnostic(ErrorCode.ERR_RequiredMembersMustBeAssignment, "{ 4, 5, 6 }").WithArguments("C.L2").WithLocation(3, 42)
+            Diagnostic(ErrorCode.ERR_RequiredMembersMustBeAssignedValue, "{ 4, 5, 6 }").WithArguments("C.L2").WithLocation(3, 42)
         );
     }
 
+    [Fact]
+    public void EnforcedRequiredMembers_NoInheritance_NestedObjectCreationWithCollectionInitializerAllowed()
+    {
+        var c = @"
+using System.Collections.Generic;
+var c = new C() { L1 = new() { 1, 2, 3 }, L2 = new() { 4, 5, 6 } };
+
+public class C
+{
+    public required List<int> L1 { get; set; }
+    public required List<int> L2;
+}
+";
+        var comp = CreateCompilationWithRequiredMembers(c);
+        CompileAndVerify(comp).VerifyDiagnostics();
+    }
+
     [Theory]
     [CombinatorialData]
     public void EnforcedRequiredMembers_Inheritance_NoneSet(bool useMetadataReference)