Case where old extensions cannot be converted to new extensions due to overloading by `ref`

[hamarb123]

Version Used: .NET Lab & Godbolt

Steps to Reproduce:

Old version:

public static class A
{
    public static void M<T>(this T value) where T : class { }
    public static void M<T>(this ref T value) where T : struct { }
}

New version:

public static class B
{
    extension<T>(T inst) where T : class
    {
        public void M() { }
    }
    extension<T>(ref T inst) where T : struct
    {
        public void M() { }
    }
}

Godbolt

.NET Lab

Diagnostic Id: error CS0111: Type 'B' already defines a member called 'M' with the same parameter types

Expected Behavior: Compiles since they get lowered to methods that don't conflict in IL, and it worked previously.

Actual Behavior: Does not compile, so cannot convert into the new extension style.

[333fred]

@jcouv

[jcouv]

From discussion with extensions WG 2025-06-27, we're planning to adjust the conflict rules to account for CLR-level constraints (ie. constraints minus notnull, tuple names and nullability annotation). That should fix this scenario

[jcouv]

This was fixed and verified by the following test:

    [Fact, WorkItem("https://github.com/dotnet/roslyn/issues/79043")]
    public void MemberNameAndSignatureConflict_35()
    {
        var src = """
new object().M();

int i = 42;
i.M();

public static class C
{
    extension<T>(T inst) where T : class
    {
        public void M() { System.Console.Write("ran1 "); }
    }
    extension<T>(ref T inst) where T : struct
    {
        public void M() { System.Console.Write("ran2"); }
    }
}
""";
        var comp = CreateCompilation(src);
        CompileAndVerify(comp, expectedOutput: "ran1 ran2").VerifyDiagnostics();
    }

Closing as fixed. Thanks for reporting this!