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Document break for negative Length tests
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‎docs/compilers/CSharp/Compiler Breaking Changes - DotNet 7.md

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## This document lists known breaking changes in Roslyn after .NET 6 all the way to .NET 7.
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1. Beginning with C# 11.0, `Length` and `Count` properties on countable and indexable types
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are assumed to be non-negative for purpose of subsumption and exhaustiveness analysis of patterns and switches.
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Those types can be used with implicit Index indexer and list patterns.
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```csharp
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void M(int[] i)
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{
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if (i is { Length: -1 }) {} // error: impossible under assumption of non-negative length
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}
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```

‎src/Compilers/CSharp/Test/Semantic/Semantics/PatternMatchingTests_ListPatterns.cs

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@@ -5023,6 +5023,160 @@ .locals init (int V_0)
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verifier.VerifyIL("C.Test2", expectedIl);
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}
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[Fact]
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public void LengthPattern_NegativeLengthTest()
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{
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var src = @"
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int[] a = null;
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_ = a is { Length: -1 }; // 1
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_ = a is { Length: -1 or 1 };
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_ = a is { Length: -1 } or { Length: 1 };
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_ = a switch // 2
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{
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{ Length: -1 } => 0, // 3
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};
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_ = a switch // 4
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{
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{ Length: -1 or 1 } => 0,
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};
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_ = a switch // 5
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{
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{ Length: -1 } or { Length: 1 } => 0,
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};
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_ = a switch // 6
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{
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{ Length: -1 } => 0, // 7
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{ Length: 1 } => 0,
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};
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";
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var comp = CreateCompilation(src);
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comp.VerifyDiagnostics(
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// (3,5): error CS8518: An expression of type 'int[]' can never match the provided pattern.
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// _ = a is { Length: -1 }; // 1
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Diagnostic(ErrorCode.ERR_IsPatternImpossible, "a is { Length: -1 }").WithArguments("int[]").WithLocation(3, 5),
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// (7,7): warning CS8509: The switch expression does not handle all possible values of its input type (it is not exhaustive). For example, the pattern '_' is not covered.
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// _ = a switch // 2
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Diagnostic(ErrorCode.WRN_SwitchExpressionNotExhaustive, "switch").WithArguments("_").WithLocation(7, 7),
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// (9,5): error CS8510: The pattern is unreachable. It has already been handled by a previous arm of the switch expression or it is impossible to match.
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// { Length: -1 } => 0, // 3
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Diagnostic(ErrorCode.ERR_SwitchArmSubsumed, "{ Length: -1 }").WithLocation(9, 5),
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// (12,7): warning CS8509: The switch expression does not handle all possible values of its input type (it is not exhaustive). For example, the pattern '{ Length: 0 }' is not covered.
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// _ = a switch // 4
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Diagnostic(ErrorCode.WRN_SwitchExpressionNotExhaustive, "switch").WithArguments("{ Length: 0 }").WithLocation(12, 7),
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// (17,7): warning CS8509: The switch expression does not handle all possible values of its input type (it is not exhaustive). For example, the pattern '{ Length: 0 }' is not covered.
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// _ = a switch // 5
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Diagnostic(ErrorCode.WRN_SwitchExpressionNotExhaustive, "switch").WithArguments("{ Length: 0 }").WithLocation(17, 7),
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// (22,7): warning CS8509: The switch expression does not handle all possible values of its input type (it is not exhaustive). For example, the pattern '{ Length: 0 }' is not covered.
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// _ = a switch // 6
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Diagnostic(ErrorCode.WRN_SwitchExpressionNotExhaustive, "switch").WithArguments("{ Length: 0 }").WithLocation(22, 7),
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// (24,5): error CS8510: The pattern is unreachable. It has already been handled by a previous arm of the switch expression or it is impossible to match.
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// { Length: -1 } => 0, // 7
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Diagnostic(ErrorCode.ERR_SwitchArmSubsumed, "{ Length: -1 }").WithLocation(24, 5)
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);
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}
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[Fact]
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public void LengthPattern_NegativeNullHandling_WithNullHandling()
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{
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var src = @"
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int[] a = null;
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_ = a is null or { Length: -1 };
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_ = a switch // 1
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{
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null => 0,
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{ Length: -1 } => 0, // 2
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};
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";
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var comp = CreateCompilation(src);
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comp.VerifyDiagnostics(
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// (5,7): warning CS8509: The switch expression does not handle all possible values of its input type (it is not exhaustive). For example, the pattern 'not null' is not covered.
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// _ = a switch // 1
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Diagnostic(ErrorCode.WRN_SwitchExpressionNotExhaustive, "switch").WithArguments("not null").WithLocation(5, 7),
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// (8,5): error CS8510: The pattern is unreachable. It has already been handled by a previous arm of the switch expression or it is impossible to match.
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// { Length: -1 } => 0, // 2
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Diagnostic(ErrorCode.ERR_SwitchArmSubsumed, "{ Length: -1 }").WithLocation(8, 5)
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);
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}
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[Fact]
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public void LengthPattern_NegativeNullHandling_DuplicateTest()
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{
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var src = @"
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int[] a = null;
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_ = a is { Length: -1 } or { Length: -1 };
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_ = a switch
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{
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{ Length: -1 } => 1,
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{ Length: -1 } => 2,
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_ => 3,
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};
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";
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var comp = CreateCompilation(src);
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comp.VerifyDiagnostics(
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// (3,5): error CS8518: An expression of type 'int[]' can never match the provided pattern.
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// _ = a is { Length: -1 } or { Length: -1 };
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Diagnostic(ErrorCode.ERR_IsPatternImpossible, "a is { Length: -1 } or { Length: -1 }").WithArguments("int[]").WithLocation(3, 5),
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// (7,5): error CS8510: The pattern is unreachable. It has already been handled by a previous arm of the switch expression or it is impossible to match.
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// { Length: -1 } => 1,
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Diagnostic(ErrorCode.ERR_SwitchArmSubsumed, "{ Length: -1 }").WithLocation(7, 5),
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// (8,5): error CS8510: The pattern is unreachable. It has already been handled by a previous arm of the switch expression or it is impossible to match.
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// { Length: -1 } => 2,
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Diagnostic(ErrorCode.ERR_SwitchArmSubsumed, "{ Length: -1 }").WithLocation(8, 5)
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);
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}
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[Fact]
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public void LengthPattern_NegativeRangeTest()
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{
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var src = @"
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int[] a = null;
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_ = a is { Length: < 0 }; // 1
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_ = a switch // 2
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{
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{ Length: < 0 } => 0, // 3
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};
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";
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var comp = CreateCompilation(src);
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comp.VerifyDiagnostics(
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// (3,5): error CS8518: An expression of type 'int[]' can never match the provided pattern.
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// _ = a is { Length: < 0 }; // 1
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Diagnostic(ErrorCode.ERR_IsPatternImpossible, "a is { Length: < 0 }").WithArguments("int[]").WithLocation(3, 5),
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// (5,7): warning CS8509: The switch expression does not handle all possible values of its input type (it is not exhaustive). For example, the pattern '_' is not covered.
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// _ = a switch // 2
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Diagnostic(ErrorCode.WRN_SwitchExpressionNotExhaustive, "switch").WithArguments("_").WithLocation(5, 7),
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// (7,5): error CS8510: The pattern is unreachable. It has already been handled by a previous arm of the switch expression or it is impossible to match.
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// { Length: < 0 } => 0, // 3
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Diagnostic(ErrorCode.ERR_SwitchArmSubsumed, "{ Length: < 0 }").WithLocation(7, 5)
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);
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}
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[Fact]
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public void LengthPattern_Switch_NegativeRangeTestByElimination()
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{
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var src = @"
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int[] a = null;
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_ = a switch
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{
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{ Length: 0 } => 1,
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{ Length: <= 0 } => 2,
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_ => 3,
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};
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";
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var comp = CreateCompilation(src);
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comp.VerifyDiagnostics(
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// (6,5): error CS8510: The pattern is unreachable. It has already been handled by a previous arm of the switch expression or it is impossible to match.
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// { Length: <= 0 } => 2,
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Diagnostic(ErrorCode.ERR_SwitchArmSubsumed, "{ Length: <= 0 }").WithLocation(6, 5)
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);
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}
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[Fact, WorkItem(51801, "https://github.com/dotnet/roslyn/issues/51801")]
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public void IndexerOverrideLacksAccessor()
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{

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