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List-patterns: list-patterns may be always-true #59478

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Feb 11, 2022
Merged

List-patterns: list-patterns may be always-true #59478

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List-patterns: list-patterns may be always-true
jcouv Feb 11, 2022
c045238
Check definite assignment
jcouv Feb 11, 2022
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2 changes: 1 addition & 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 @@ -71,13 +71,13 @@ private BoundExpression MakeIsPatternExpression(
{
case BoundConstantPattern _:
case BoundITuplePattern _:
case BoundListPattern:
// these patterns can fail in practice
throw ExceptionUtilities.Unreachable;
case BoundRelationalPattern _:
case BoundTypePattern _:
case BoundNegatedPattern _:
case BoundBinaryPattern _:
case BoundListPattern:
Debug.Assert(expression.Type is object);
diagnostics.Add(ErrorCode.WRN_IsPatternAlways, node.Location, expression.Type);
break;
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33 changes: 33 additions & 0 deletions src/Compilers/CSharp/Test/Semantic/Semantics/PatternMatchingTests_ListPatterns.cs
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Expand Up @@ -2761,6 +2761,39 @@ public static void Test5<T>(T t) where T : IIndexable, ICountableViaLength, ICou
);
}

[Fact, WorkItem(59466, "https://github.com/dotnet/roslyn/issues/59466")]
public void AlwaysTruePattern()
{
var source = @"
_ = new S() is [..var y];
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I think you could verify y is definitely assigned after this statement.

_ = new S() is [..];
_ = new S() is [..[..]];
_ = new S() is not [..];

struct S
{
public int Length => 1;
public int this[int i] => 42;
public S this[System.Range r] => default;
}
";
var comp = CreateCompilationWithIndexAndRangeAndSpan(source);
comp.VerifyEmitDiagnostics(
// (2,5): warning CS8794: An expression of type 'S' always matches the provided pattern.
// _ = new S() is [..var y];
Diagnostic(ErrorCode.WRN_IsPatternAlways, "new S() is [..var y]").WithArguments("S").WithLocation(2, 5),
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Is the expectation here that user should simplify to var y?

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The user should simply write is var y (which would not warn).
List and slice patterns make assumptions about collections being well-behaved. If you grab the entire range, you should get the same collection, so that dicing wasn't necessary.

// (3,5): warning CS8794: An expression of type 'S' always matches the provided pattern.
// _ = new S() is [..];
Diagnostic(ErrorCode.WRN_IsPatternAlways, "new S() is [..]").WithArguments("S").WithLocation(3, 5),
// (4,5): warning CS8794: An expression of type 'S' always matches the provided pattern.
// _ = new S() is [..[..]];
Diagnostic(ErrorCode.WRN_IsPatternAlways, "new S() is [..[..]]").WithArguments("S").WithLocation(4, 5),
// (5,5): error CS8518: An expression of type 'S' can never match the provided pattern.
// _ = new S() is not [..];
Diagnostic(ErrorCode.ERR_IsPatternImpossible, "new S() is not [..]").WithArguments("S").WithLocation(5, 5)
);
}

[Fact]
public void ListPattern_ValEscape()
{
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