diagnostic for override in Interface should suggest using explicit interface implementation instead · dotnet/roslyn#39940

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Area-CompilersConcept-Diagnostic ClarityFeature - Default Interface ImplFeature Requesthelp wanted

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Version Used: VS 16.4 preview 4

Steps to Reproduce:

The following code fails to compile:

public interface I {
    abstract void M();
}

public interface I1 : I {
    override void M() {}
}

public class C : I1 {}

The correct code should be:

public interface I {
    abstract void M();
}

public interface I1 : I {
    void I.M() {}
}

public class C : I1 {}

However it's not immediately obvious that's the case, and there's nothing pointing you to use explicit interface implementation instead.

Expected Behavior:

DIagnostic should be:

error CS0106: The modifier 'override' is not valid for this item. Use explicit interface implementation void I.M instead.

Ideally there should also be a code fix for this.

Actual Behavior:

error CS0106: The modifier 'override' is not valid for this item

Research direction: Investigate how diagnostic CS0106 is emitted for interface methods with override modifier. Look at the error message formatting in the compiler and consider adding a suggestion for explicit interface implementation. Then implement a code fix to transform override to explicit implementation.
Tech stack: csharp
Domain: backend
Issue type: Bug
Difficulty: 3
Estimated time: 1-3 hours
Activity status: Fresh
Clarity: Clear
Prerequisites: C#Visual Studio SDK
Newbie friendliness: 50