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Improve async iterator diagnostic by removing redundant error #81282

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Improve async iterator diagnostic by removing redundant error #81282

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Initial plan
Copilot Nov 16, 2025
4ea5dc9
Improve diagnostic for async iterators without async keyword
Copilot Nov 16, 2025
f79a4b6
Update src/Compilers/CSharp/Test/Emit/CodeGen/CodeGenAsyncIteratorTes…
CyrusNajmabadi Nov 16, 2025
386f33a
Update src/Compilers/CSharp/Test/Emit/CodeGen/CodeGenAsyncIteratorTes…
CyrusNajmabadi Nov 16, 2025
3de3769
Use ERR_BadAwaitWithoutAsync instead of new error code
Copilot Nov 16, 2025
1a57671
Update src/Compilers/CSharp/Portable/Binder/Binder_Await.cs
CyrusNajmabadi Nov 16, 2025
cac74c6
Use raw string literals in test code
Copilot Nov 16, 2025
b7d326e
Fix formatting: remove trailing spaces
Copilot Nov 16, 2025
3b78eef
Revert existing test to use verbatim strings
Copilot Nov 16, 2025
a5766a3
Don't report await error when iterator already reports missing async
Copilot Nov 17, 2025
5b4c38e
Use shared IsAsyncStreamInterface method and improve comment
Copilot Nov 17, 2025
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18 changes: 15 additions & 3 deletions src/Compilers/CSharp/Portable/Binder/Binder_Await.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -185,9 +185,21 @@ private bool ReportBadAwaitWithoutAsync(SyntaxNodeOrToken nodeOrToken, BindingDi
}
else
{
info = method.ReturnsVoid ?
new CSDiagnosticInfo(ErrorCode.ERR_BadAwaitWithoutVoidAsyncMethod) :
new CSDiagnosticInfo(ErrorCode.ERR_BadAwaitWithoutAsyncMethod, method.ReturnType);
if (method.ReturnsVoid)
{
info = new CSDiagnosticInfo(ErrorCode.ERR_BadAwaitWithoutVoidAsyncMethod);
}
else if (method.IsIterator && InMethodBinder.IsAsyncStreamInterface(Compilation, method.RefKind, method.ReturnType))
{
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@CyrusNajmabadi CyrusNajmabadi Nov 17, 2025 •
edited by jcouv
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@copilot mention which method reports this ERR_IteratorMustBeAsync error. Also, if possible, can we share code between that location and this location to ensure that we're checking all the same conditions to report an error there and not report an error here? #Closed

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Copilot AI Nov 17, 2025

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Done in 5b4c38e. Now using the shared InMethodBinder.IsAsyncStreamInterface method to ensure we check the same conditions, and updated the comment to mention ExecutableCodeBinder.ValidateIteratorMethod as the location that reports the error.

// For async iterators, we don't report an error on await because ERR_IteratorMustBeAsync
// is already reported by ExecutableCodeBinder.ValidateIteratorMethod, which tells the user to add 'async'.
// Reporting a second error here would be redundant.
return false;
}
else
{
info = new CSDiagnosticInfo(ErrorCode.ERR_BadAwaitWithoutAsyncMethod, method.ReturnType);
}
}
break;
}
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25 changes: 20 additions & 5 deletions src/Compilers/CSharp/Test/Emit/CodeGen/CodeGenAsyncIteratorTests.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -1996,13 +1996,28 @@ static System.Collections.Generic.IAsyncEnumerator<int> M(int value)
comp.VerifyDiagnostics(
// (4,61): error CS8403: Method 'C.M(int)' with an iterator block must be 'async' to return 'IAsyncEnumerator<int>'
// static System.Collections.Generic.IAsyncEnumerator<int> M(int value)
Diagnostic(ErrorCode.ERR_IteratorMustBeAsync, "M").WithArguments("C.M(int)", "System.Collections.Generic.IAsyncEnumerator<int>").WithLocation(4, 61),
// (7,9): error CS4032: The 'await' operator can only be used within an async method. Consider marking this method with the 'async' modifier and changing its return type to 'Task<IAsyncEnumerator<int>>'.
// await System.Threading.Tasks.Task.CompletedTask;
Diagnostic(ErrorCode.ERR_BadAwaitWithoutAsyncMethod, "await System.Threading.Tasks.Task.CompletedTask").WithArguments("System.Collections.Generic.IAsyncEnumerator<int>").WithLocation(7, 9)
Diagnostic(ErrorCode.ERR_IteratorMustBeAsync, "M").WithArguments("C.M(int)", "System.Collections.Generic.IAsyncEnumerator<int>").WithLocation(4, 61)
);
}

// This error message is rather poor. Tracked by https://github.com/dotnet/roslyn/issues/31113
[Fact, WorkItem("https://github.com/dotnet/roslyn/issues/31113")]
public void AsyncIteratorReturningEnumerable_WithoutAsync()
{
string source = """
class C
{
static System.Collections.Generic.IAsyncEnumerable<int> M(int value)
{
yield return value;
await System.Threading.Tasks.Task.CompletedTask;
}
}
""";
var comp = CreateCompilationWithAsyncIterator(source);
comp.VerifyDiagnostics(
// (3,61): error CS8403: Method 'C.M(int)' with an iterator block must be 'async' to return 'IAsyncEnumerable<int>'
// static System.Collections.Generic.IAsyncEnumerable<int> M(int value)
Diagnostic(ErrorCode.ERR_IteratorMustBeAsync, "M").WithArguments("C.M(int)", "System.Collections.Generic.IAsyncEnumerable<int>").WithLocation(3, 61));
}

[Fact]
Expand Down