List patterns: subsumption checking

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

@@ -2,6 +2,7 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+using System;
 using System.Collections.Immutable;
 using System.Diagnostics;
 using System.Diagnostics.CodeAnalysis;
@@ -341,11 +342,9 @@ private bool TryPerformPatternIndexerLookup(
             BindingDiagnosticBag diagnostics)
         {
             Debug.Assert(!receiverType.IsErrorType());
-
             indexerAccess = null;
             patternSymbol = null;
             lengthProperty = null;
-
             bool found;
             CompoundUseSiteInfo<AssemblySymbol> useSiteInfo = GetNewCompoundUseSiteInfo(diagnostics);
             var bindingDiagnostics = BindingDiagnosticBag.GetInstance(diagnostics);
@@ -1437,6 +1436,7 @@ private ImmutableArray<BoundPropertySubpattern> BindPropertyPatternClause(
                 PatternSyntax pattern = p.Pattern;
                 BoundPropertySubpatternMember? member;
                 TypeSymbol memberType;
+                bool isLengthOrCount = false;
                 if (expr == null)
                 {
                     if (!hasErrors)
@@ -1450,10 +1450,22 @@ private ImmutableArray<BoundPropertySubpattern> BindPropertyPatternClause(
                 {
                     member = LookupMembersForPropertyPattern(inputType, expr, diagnostics, ref hasErrors);
                     memberType = member.Type;
+                    // If we're dealing with the member that makes the type countable, and the type is also indexable, then it will be assumed to always return a non-negative value
+                    if (memberType.SpecialType == SpecialType.System_Int32 &&
+                        member.Symbol is { Name: WellKnownMemberNames.LengthPropertyName or WellKnownMemberNames.CountPropertyName, Kind: SymbolKind.Property } memberSymbol)
+                    {
+                        TypeSymbol receiverType = member.Receiver?.Type ?? inputType;
+                        if (!receiverType.IsErrorType())
+                        {
+                            isLengthOrCount = receiverType.IsSZArray()
+                                ? ReferenceEquals(memberSymbol, Compilation.GetSpecialTypeMember(SpecialMember.System_Array__Length))
+                                : TryPerformPatternIndexerLookup(node, receiverType, argIsIndex: true, indexerAccess: out _, patternSymbol: out _, lengthProperty: out _, BindingDiagnosticBag.Discarded);
+                        }
+                    }
                 }
 
                 BoundPattern boundPattern = BindPattern(pattern, memberType, GetValEscape(memberType, inputValEscape), permitDesignations, hasErrors, diagnostics);
-                builder.Add(new BoundPropertySubpattern(p, member, boundPattern));
+                builder.Add(new BoundPropertySubpattern(p, member, isLengthOrCount, boundPattern));
             }
 
             return builder.ToImmutableAndFree();

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

@@ -32,15 +32,11 @@ private Tests MakeTestsAndBindingsForListPattern(BoundDagTemp input, BoundListPa
                      subpatterns[0] is BoundSlicePattern { Pattern: null })
             {
                 // If `..` is the only pattern in the list, bail. This is a no-op and we don't need to match anything further.
-                // If there's a subpattern, we're going to need the length value to extract a slice for the input,
-                // in which case we will test the length even if there is no other patterns in the list.
-                // i.e. the length is required to be non-negative for the match to be correct.
             }
             else
             {
                 Debug.Assert(list.LengthProperty is not null);
-
-                var lengthEvaluation = new BoundDagPropertyEvaluation(syntax, list.LengthProperty, input);
+                var lengthEvaluation = new BoundDagPropertyEvaluation(syntax, list.LengthProperty, isLengthOrCount: true, input);
                 tests.Add(new Tests.One(lengthEvaluation));
                 var lengthTemp = new BoundDagTemp(syntax, _compilation.GetSpecialType(SpecialType.System_Int32), lengthEvaluation);
                 tests.Add(new Tests.One(list.HasSlice

src/Compilers/CSharp/Portable/BoundTree/BoundDagEvaluation.cs

@@ -10,13 +10,22 @@ namespace Microsoft.CodeAnalysis.CSharp
 {
     partial class BoundDagEvaluation
     {
-        public override bool Equals([NotNullWhen(true)] object? obj) => obj is BoundDagEvaluation other && this.Equals(other);
-        public virtual bool Equals(BoundDagEvaluation other)
+        public sealed override bool Equals([NotNullWhen(true)] object? obj) => obj is BoundDagEvaluation other && this.Equals(other);
+        public bool Equals(BoundDagEvaluation other)
         {
             return this == other ||
-                this.Kind == other.Kind &&
-                this.Input.Equals(other.Input) &&
-                Symbol.Equals(this.Symbol, other.Symbol, TypeCompareKind.AllIgnoreOptions);
+                this.IsEquivalentTo(other) &&
+                this.Input.Equals(other.Input);
+        }
+
+        /// <summary>
+        /// Check if this is equivalent to the <paramref name="other"/> node, ignoring the input.
+        /// </summary>
+        public virtual bool IsEquivalentTo(BoundDagEvaluation other)
+        {
+            return this == other ||
+               this.Kind == other.Kind &&
+               Symbol.Equals(this.Symbol, other.Symbol, TypeCompareKind.AllIgnoreOptions);
         }
 
         private Symbol? Symbol
@@ -32,6 +41,7 @@ private Symbol? Symbol
                     BoundDagIndexEvaluation e => e.Property,
                     BoundDagSliceEvaluation e => (Symbol?)e.SliceMethod ?? e.IndexerAccess?.Indexer,
                     BoundDagIndexerEvaluation e => e.IndexerSymbol ?? e.IndexerAccess?.Indexer,
+                    BoundDagAssignmentEvaluation => null,
                     _ => throw ExceptionUtilities.UnexpectedValue(this.Kind)
                 };
             }
@@ -79,37 +89,42 @@ internal string GetOutputTempDebuggerDisplay()
     partial class BoundDagIndexEvaluation
     {
         public override int GetHashCode() => base.GetHashCode() ^ this.Index;
-        public override bool Equals(BoundDagEvaluation obj)
+        public override bool IsEquivalentTo(BoundDagEvaluation obj)
         {
-            return this == obj ||
-                base.Equals(obj) &&
-                // base.Equals checks the kind field, so the following cast is safe
+            return base.IsEquivalentTo(obj) &&
+                // base.IsEquivalentTo checks the kind field, so the following cast is safe
                 this.Index == ((BoundDagIndexEvaluation)obj).Index;
         }
     }
 
     partial class BoundDagIndexerEvaluation
     {
         public override int GetHashCode() => base.GetHashCode() ^ this.Index;
-        public override bool Equals(BoundDagEvaluation obj)
+        public override bool IsEquivalentTo(BoundDagEvaluation obj)
         {
-            return this == obj ||
-                base.Equals(obj) &&
-                // base.Equals checks the kind field, so the following cast is safe
+            return base.IsEquivalentTo(obj) &&
                 this.Index == ((BoundDagIndexerEvaluation)obj).Index;
         }
     }
 
     partial class BoundDagSliceEvaluation
     {
         public override int GetHashCode() => base.GetHashCode() ^ this.StartIndex ^ this.EndIndex;
-        public override bool Equals(BoundDagEvaluation obj)
+        public override bool IsEquivalentTo(BoundDagEvaluation obj)
         {
-            return this == obj ||
-                base.Equals(obj) &&
-                // base.Equals checks the kind field, so the following cast is safe
+            return base.IsEquivalentTo(obj) &&
                 (BoundDagSliceEvaluation)obj is var e &&
                 this.StartIndex == e.StartIndex && this.EndIndex == e.EndIndex;
         }
     }
+
+    partial class BoundDagAssignmentEvaluation
+    {
+        public override int GetHashCode() => Hash.Combine(base.GetHashCode(), this.Target.GetHashCode());
+        public override bool IsEquivalentTo(BoundDagEvaluation obj)
+        {
+            return base.IsEquivalentTo(obj) &&
+                this.Target.Equals(((BoundDagAssignmentEvaluation)obj).Target);
+        }
+    }
 }

src/Compilers/CSharp/Portable/BoundTree/BoundDagTemp.cs

@@ -18,15 +18,26 @@ partial class BoundDagTemp
         /// </summary>
         public bool IsOriginalInput => this.Source is null;
 
-        public static BoundDagTemp ForOriginalInput(SyntaxNode syntax, TypeSymbol type) => new BoundDagTemp(syntax, type, null, 0);
+        public static BoundDagTemp ForOriginalInput(SyntaxNode syntax, TypeSymbol type) => new BoundDagTemp(syntax, type, source: null, 0);
 
         public override bool Equals(object? obj) => obj is BoundDagTemp other && this.Equals(other);
 
         public bool Equals(BoundDagTemp other)
         {
-            return other is { } &&
+            return
                 this.Type.Equals(other.Type, TypeCompareKind.AllIgnoreOptions) &&
-                object.Equals(this.Source, other.Source) && this.Index == other.Index;
+                object.Equals(this.Source, other.Source) &&
+                this.Index == other.Index;
+        }
+
+        /// <summary>
+        /// Check if this is equivalent to the <paramref name="other"/> node, ignoring the source.
+        /// </summary>
+        public bool IsEquivalentTo(BoundDagTemp other)
+        {
+            return
+                this.Type.Equals(other.Type, TypeCompareKind.AllIgnoreOptions) &&
+                this.Index == other.Index;
         }
 
         public override int GetHashCode()

src/Compilers/CSharp/Portable/BoundTree/BoundDagTest.cs

@@ -21,6 +21,8 @@ private bool Equals(BoundDagTest? other)
                 return false;
             if (this == other)
                 return true;
+            if (!this.Input.Equals(other.Input))
+                return false;
 
             switch (this, other)
             {
@@ -71,6 +73,8 @@ public override int GetHashCode()
                     return result;
                 case BoundDagIndexEvaluation i:
                     return $"{i.GetOutputTempDebuggerDisplay()} = {i.Input.GetDebuggerDisplay()}[{i.Index}]";
+                case BoundDagIndexerEvaluation i:
+                    return $"{i.GetOutputTempDebuggerDisplay()} = {i.Input.GetDebuggerDisplay()}[{i.Index}]";
                 case BoundDagEvaluation e:
                     return $"{e.GetOutputTempDebuggerDisplay()} = {e.Kind}({e.Input.GetDebuggerDisplay()})";
                 case BoundDagTypeTest b:

src/Compilers/CSharp/Portable/BoundTree/BoundNodes.xml

@@ -1429,6 +1429,7 @@
   </Node>
   <Node Name="BoundDagPropertyEvaluation" Base="BoundDagEvaluation">
     <Field Name="Property" Type="PropertySymbol" Null="disallow"/>
+    <Field Name="IsLengthOrCount" Type="bool"/>
   </Node>
   <Node Name="BoundDagIndexEvaluation" Base="BoundDagEvaluation">
     <Field Name="Property" Type="PropertySymbol" Null="disallow"/>
@@ -1450,6 +1451,9 @@
     <Field Name="IndexerAccess" Type="BoundIndexerAccess?"/>
     <Field Name="SliceMethod" Type="MethodSymbol?"/>
   </Node>
+  <Node Name="BoundDagAssignmentEvaluation" Base="BoundDagEvaluation">
+    <Field Name="Target" Type="BoundDagTemp"/>
+  </Node>
 
   <Node Name="BoundSwitchSection" Base="BoundStatementList">
     <Field Name="Locals" Type="ImmutableArray&lt;LocalSymbol&gt;"/>
@@ -2170,6 +2174,7 @@
   <Node Name="BoundPropertySubpattern" Base="BoundSubpattern">
     <!-- The property or field access in a property pattern. -->
     <Field Name="Member" Type="BoundPropertySubpatternMember?"/>
+    <Field Name="IsLengthOrCount" Type="bool"/>
   </Node>
   <Node Name="BoundPropertySubpatternMember" Base="BoundNode">
     <Field Name="Receiver" Type="BoundPropertySubpatternMember?"/>

src/Compilers/CSharp/Portable/FlowAnalysis/NullableWalker_Patterns.cs

@@ -503,6 +503,8 @@ public PossiblyConditionalState Clone()
                                 case BoundDagSliceEvaluation e:
                                     addTemp(e, e.SliceType);
                                     break;
+                                case BoundDagAssignmentEvaluation e:
+                                    break;
                                 default:
                                     throw ExceptionUtilities.UnexpectedValue(p.Evaluation.Kind);
                             }