Unexpected semantic information from bad method group conversion in local declaration

[jcouv]

There's also similar issue in assignment and possibly in invocations. See tests that refer to this issue.

        [Fact, WorkItem("https://github.com/dotnet/roslyn/issues/75833")]
        public void GetSymbolInfo_ImplicitUserDefinedConversionOnMethodGroup_InLocalDeclaration()
        {
            var src = """
public class C
{
    public static void M()
    {
        C x = C.Test;
    }

    public static int Test() => 1;

    public static implicit operator C(System.Func<int> intDelegate)
    {
        return new C();
    }
}
""";
            var comp = CreateCompilation(src);
            comp.VerifyEmitDiagnostics(
                // (5,17): error CS0428: Cannot convert method group 'Test' to non-delegate type 'C'. Did you intend to invoke the method?
                //         C x = C.Test;
                Diagnostic(ErrorCode.ERR_MethGrpToNonDel, "Test").WithArguments("Test", "C").WithLocation(5, 17));

            var tree = comp.SyntaxTrees.Single();
            var model = comp.GetSemanticModel(tree);
            var memberAccess = GetSyntax<MemberAccessExpressionSyntax>(tree, "C.Test");
            Assert.Equal("C C.op_Implicit(System.Func<System.Int32> intDelegate)", model.GetSymbolInfo(memberAccess).Symbol.ToTestDisplayString());  // Unexpected: Should be null
            var conversion = model.GetConversion(memberAccess);
            Assert.Equal(ConversionKind.ExplicitUserDefined, conversion.Kind); // Unexpected: Should be NoConversion or possibly Identity for error case
            Assert.Equal(ConversionKind.MethodGroup, conversion.UserDefinedFromConversion.Kind);
            Assert.Equal(ConversionKind.Identity, conversion.UserDefinedToConversion.Kind);
        }

        [Fact, WorkItem("https://github.com/dotnet/roslyn/issues/75833")]
        public void GetSymbolInfo_ImplicitUserDefinedConversionOnMethodGroup_WithToConversion_InLocalDeclaration()
        {
            var src = """
public struct C
{
    public static void M()
    {
        C? x = C.Test;
    }

    public static int Test() => 1;

    public static implicit operator C(System.Func<int> intDelegate)
    {
        return new C();
    }
}
""";
            var comp = CreateCompilation(src);
            comp.VerifyEmitDiagnostics(
                // (5,18): error CS0428: Cannot convert method group 'Test' to non-delegate type 'C?'. Did you intend to invoke the method?
                //         C? x = C.Test;
                Diagnostic(ErrorCode.ERR_MethGrpToNonDel, "Test").WithArguments("Test", "C?").WithLocation(5, 18));

            var tree = comp.SyntaxTrees.Single();
            var model = comp.GetSemanticModel(tree);
            var memberAccess = GetSyntax<MemberAccessExpressionSyntax>(tree, "C.Test");
            Assert.Null(model.GetSymbolInfo(memberAccess).Symbol);
            var conversion = model.GetConversion(memberAccess);
            Assert.Equal(ConversionKind.ExplicitUserDefined, conversion.Kind); // Unexpected: Should be NoConversion or possibly Identity for error case
            Assert.Equal(ConversionKind.MethodGroup, conversion.UserDefinedFromConversion.Kind);
            Assert.Equal(ConversionKind.Identity, conversion.UserDefinedToConversion.Kind);
        }

[jaredpar]

How impactful is this issue? Is there a reason we didn't address it with the other fix for method group conversions?

[jcouv]

This is less impactful than the scenario that was fixed because this is an error scenario. Also, this scenario doesn't go through the codepath that was affected in that fix/PR, so it was better to separate.