If Length and Count are both present, only the former is assumed to b…

…e non-negative (#57268)
dotnet · Oct 22, 2021 · 3fc766e · 3fc766e
1 parent 039cc3d
commit 3fc766e
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Showing 2 changed files with 8 additions and 12 deletions.
diff --git a/src/Compilers/CSharp/Portable/Binder/Binder_Patterns.cs b/src/Compilers/CSharp/Portable/Binder/Binder_Patterns.cs
@@ -1458,7 +1458,8 @@ private ImmutableArray<BoundPropertySubpattern> BindPropertyPatternClause(
                         {
                             isLengthOrCount = receiverType.IsSZArray()
                                 ? ReferenceEquals(memberSymbol, Compilation.GetSpecialTypeMember(SpecialMember.System_Array__Length))
-                                : TryPerformPatternIndexerLookup(node, receiverType, argIsIndex: true, indexerAccess: out _, patternSymbol: out _, lengthProperty: out _, BindingDiagnosticBag.Discarded);
+                                : TryPerformPatternIndexerLookup(node, receiverType, argIsIndex: true, indexerAccess: out _, patternSymbol: out _, out PropertySymbol? lengthProperty, BindingDiagnosticBag.Discarded) &&
+                                  memberSymbol.Equals(lengthProperty, TypeCompareKind.ConsiderEverything); // If Length and Count are both present, only the former is assumed to be non-negative.
                         }
                     }
                 }

diff --git a/src/Compilers/CSharp/Test/Semantic/Semantics/PatternMatchingTests_ListPatterns.cs b/src/Compilers/CSharp/Test/Semantic/Semantics/PatternMatchingTests_ListPatterns.cs
@@ -1608,9 +1608,9 @@ public static void Test4<T>(T t) where T : IIndexable, ICountableViaLength
     public static void Test5<T>(T t) where T : IIndexable, ICountableViaLength, ICountableViaCount
     {
         _ = t is { Length: -1 }; // 5
-        _ = t is { Count: -1 }; // 6
-        _ = new { t } is { t.Count: -1 }; // 7
-        _ = new { t } is { t.Length: -1 }; // 8
+        _ = t is { Count: -1 };
+        _ = new { t } is { t.Length: -1 }; // 6
+        _ = new { t } is { t.Count: -1 };
     }
 }
 ";
@@ -1631,15 +1631,10 @@ public static void Test5<T>(T t) where T : IIndexable, ICountableViaLength, ICou
             // (38,13): error CS8518: An expression of type 'T' can never match the provided pattern.
             //         _ = t is { Length: -1 }; // 5
             Diagnostic(ErrorCode.ERR_IsPatternImpossible, "t is { Length: -1 }").WithArguments("T").WithLocation(38, 13),
-            // (39,13): error CS8518: An expression of type 'T' can never match the provided pattern.
-            //         _ = t is { Count: -1 }; // 6
-            Diagnostic(ErrorCode.ERR_IsPatternImpossible, "t is { Count: -1 }").WithArguments("T").WithLocation(39, 13),
             // (40,13): error CS8518: An expression of type '<anonymous type: T t>' can never match the provided pattern.
-            //         _ = new { t } is { t.Count: -1 }; // 7
-            Diagnostic(ErrorCode.ERR_IsPatternImpossible, "new { t } is { t.Count: -1 }").WithArguments("<anonymous type: T t>").WithLocation(40, 13),
-            // (41,13): error CS8518: An expression of type '<anonymous type: T t>' can never match the provided pattern.
-            //         _ = new { t } is { t.Length: -1 }; // 8
-            Diagnostic(ErrorCode.ERR_IsPatternImpossible, "new { t } is { t.Length: -1 }").WithArguments("<anonymous type: T t>").WithLocation(41, 13));
+            //         _ = new { t } is { t.Length: -1 }; // 6
+            Diagnostic(ErrorCode.ERR_IsPatternImpossible, "new { t } is { t.Length: -1 }").WithArguments("<anonymous type: T t>").WithLocation(40, 13)
+            );
     }
 
     [Fact]