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If Length and Count are both present, only the former is assumed to be non-negative #57268

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Oct 22, 2021
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If Length and Count are both present, only the former is assumed to be non-negative #57268

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3 changes: 2 additions & 1 deletion src/Compilers/CSharp/Portable/Binder/Binder_Patterns.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -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.
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📝 Linking to #53891 (comment) if more investigation is needed.

}
}
}
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17 changes: 6 additions & 11 deletions src/Compilers/CSharp/Test/Semantic/Semantics/PatternMatchingTests_ListPatterns.cs
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Expand Up @@ -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 };
}
}
";
Expand All @@ -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]
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