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Suppress ERR_BadRetType and ERR_MethDelegateMismatch for delegate conversions with omitted type arguments #81278

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Suppress ERR_BadRetType and ERR_MethDelegateMismatch for delegate conversions with omitted type arguments #81278

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1842f92
Initial plan
Copilot Nov 16, 2025
913b839
Fix: Suppress ERR_BadRetType when return type contains type parameters
Copilot Nov 16, 2025
78786f7
Simplify fix to check ContainsErrorType only
Copilot Nov 16, 2025
8e1be16
Remove Binder_Conversions.cs change and handle ERR_MethDelegateMismat…
Copilot Nov 16, 2025
dc326a1
Fix formatting: remove trailing whitespace from blank lines
Copilot Nov 16, 2025
45ce1e2
Simplify
CyrusNajmabadi Nov 17, 2025
878ac15
Add test and fix for parameter types with omitted type arguments
Copilot Nov 17, 2025
703d298
Use raw string literal in test
Copilot Nov 17, 2025
14d1ba2
Add WorkItem attribute to test
Copilot Nov 17, 2025
f5ec480
Update src/Compilers/CSharp/Portable/Binder/Semantics/Conversions/Con…
CyrusNajmabadi Nov 17, 2025
c7ac7e5
Indent raw string literal and rename Foo to Goo
Copilot Nov 17, 2025
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8 changes: 7 additions & 1 deletion src/Compilers/CSharp/Portable/Binder/Semantics/Conversions/Conversions.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -382,7 +382,13 @@ public static bool ReportDelegateOrFunctionPointerMethodGroupDiagnostics(Binder
typeContainingConstructor: null, delegateTypeBeingInvoked: null,
isMethodGroupConversion: true, returnRefKind: invokeMethodOpt?.RefKind, delegateOrFunctionPointerType: targetType);

hasErrors = overloadDiagnostics.HasAnyErrors();
// Even if no diagnostics were reported, check if any candidate method has error types
// in its return type or parameters. This can happen when ERR_BadRetType or similar errors
// are suppressed for methods with omitted type arguments.
hasErrors = overloadDiagnostics.HasAnyErrors()
|| result.Results.Any(static candidate => candidate.Member.ReturnType.ContainsErrorType()
|| candidate.Member.Parameters.Any(static p => p.Type.ContainsErrorType()));

diagnostics.AddRangeAndFree(overloadDiagnostics);
}
}
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8 changes: 7 additions & 1 deletion ...Compilers/CSharp/Portable/Binder/Semantics/OverloadResolution/OverloadResolutionResult.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -726,7 +726,13 @@ private bool HadReturnMismatch(Location location, BindingDiagnosticBag diagnosti
if (!mismatch.IsNull)
{
var method = (MethodSymbol)(Symbol)mismatch.Member;
diagnostics.Add(ErrorCode.ERR_BadRetType, location, method, method.ReturnType);
// Only report ERR_BadRetType if the return type is well-defined.
// If the return type is an error type (from omitted type arguments like Method<>),
// there's already a more specific error reported elsewhere (e.g., ERR_OmittedTypeArgument).
if (!method.ReturnType.ContainsErrorType())
{
diagnostics.Add(ErrorCode.ERR_BadRetType, location, method, method.ReturnType);
}
return true;
}

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29 changes: 26 additions & 3 deletions src/Compilers/CSharp/Test/Semantic/Semantics/BindingTests.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -3860,9 +3860,6 @@ public void Test()
// (33,51): error CS8389: Omitting the type argument is not allowed in the current context
// System.Func<object> delegateConversion1 = "string literal".ExtensionMethod1<>;
Diagnostic(ErrorCode.ERR_OmittedTypeArgument, @"""string literal"".ExtensionMethod1<>").WithLocation(33, 51),
// (33,51): error CS0407: '? FooExtensions.ExtensionMethod1<?>(object)' has the wrong return type
// System.Func<object> delegateConversion1 = "string literal".ExtensionMethod1<>;
Diagnostic(ErrorCode.ERR_BadRetType, @"""string literal"".ExtensionMethod1<>").WithArguments("FooExtensions.ExtensionMethod1<?>(object)", "?").WithLocation(33, 51),
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@CyrusNajmabadi CyrusNajmabadi Nov 16, 2025

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@copilot you made two changes to the code. Are there tests demonstrating the effect of each change?

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Copilot AI Nov 16, 2025

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The existing test at line 3785 (System.Func<object> delegateConversion1 = "string literal".ExtensionMethod1<>;) exercises both changes. During method group conversion to a delegate type:

  1. First, overload resolution is performed, which may call HadReturnMismatch in OverloadResolutionResult.cs if the return type doesn't match
  2. Then, if a method is selected, MethodIsCompatibleWithDelegateOrFunctionPointer in Binder_Conversions.cs verifies the delegate compatibility

Both code paths suppress ERR_BadRetType when the return type contains error types (from omitted type arguments). The test verifies that the confusing ERR_BadRetType error with ? symbols is no longer reported for this scenario, confirming both changes work correctly together.

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@CyrusNajmabadi CyrusNajmabadi Nov 17, 2025

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@copilot this is a good update that validates the case where the return type contains an error type.

Can you add a test after this that is a trimmed down version of this, except that the extension method has this form:

public static void ExtensionMethod1<T>(this object obj, T t) { } and assigns to Action<object>.

Let's see if we have a similar problem where we report ? for the parameter type. If not, then we don't need to do anything more. If we see a similar problem, then we should update our checks accordingly to check parameters as well as return type.

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Copilot AI Nov 17, 2025

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Added test in commit 878ac15. The same issue exists for parameter types - ERR_MethDelegateMismatch was being reported. Extended the fix to also check for error types in method parameters, not just return types. Both tests now pass.

// (34,51): error CS8389: Omitting the type argument is not allowed in the current context
// System.Func<object> delegateConversion2 = "string literal".ExtensionMethod2<>;
Diagnostic(ErrorCode.ERR_OmittedTypeArgument, @"""string literal"".ExtensionMethod2<>").WithLocation(34, 51),
Expand All @@ -3871,6 +3868,32 @@ public void Test()
Diagnostic(ErrorCode.ERR_NoSuchMemberOrExtension, "ExtensionMethod2<>").WithArguments("string", "ExtensionMethod2").WithLocation(34, 68));
}

[Fact, WorkItem("https://github.com/dotnet/roslyn/issues/16041")]
public void MissingTypeArgumentInGenericExtensionMethod_ParameterType()
{
var source =
"""
public static class GooExtensions
{
public static void ExtensionMethod1<T>(this object obj, T t) { }
}

public class Class1
{
public void Test()
{
System.Action<object> delegateConversion1 = "literal".ExtensionMethod1<>;
}
}
""";
var compilation = CreateCompilationWithMscorlib40AndSystemCore(source);

compilation.VerifyDiagnostics(
// (10,53): error CS8389: Omitting the type argument is not allowed in the current context
// System.Action<object> delegateConversion1 = "literal".ExtensionMethod1<>;
Diagnostic(ErrorCode.ERR_OmittedTypeArgument, @"""literal"".ExtensionMethod1<>").WithLocation(10, 53));
}

[WorkItem(22757, "https://github.com/dotnet/roslyn/issues/22757")]
[Fact]
public void MethodGroupConversionNoReceiver()
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