Default interface implementation of static abstract operator is not resolved.

[gurustron]

Description

Not sure if this a bug though and maybe this is better addressed in the csharplang repo.

When interface provides implementation for IEqualityOperators.== operator the implementation is not resolved unless via generic indirection.

Original SO question

Reproduction Steps

public interface IAutoEquatable<T> : IEquatable<T>, IEqualityOperators<T, T, bool> where T : IAutoEquatable<T>
{
    static bool IEqualityOperators<T, T, bool>.operator ==(T? left, T? right)
    {
        Console.Write("ieop.== ");
        if (ReferenceEquals(left, null) && ReferenceEquals(right, null))
        {
            return true;
        }

        if (ReferenceEquals(left, null) || ReferenceEquals(right, null))
        {
            return false;
        }

        return left.Equals(right);
    }

    static bool IEqualityOperators<T, T, bool>.operator !=(T? left, T? right)
    {
        return !(left == right);
    }
}

class Foo : IAutoEquatable<Foo>
{
    public bool Equals(Foo? other)
    {
        Console.Write("eq ");
        return true;
    }
}

And usage:

// 1
Do<Foo>();
// 2
Do1<Foo>();
// 3
Console.WriteLine(new Foo() == new Foo());
// 4
IEqualityOperators<Foo, Foo, bool> foo1 = new Foo(); 
IEqualityOperators<Foo, Foo, bool> foo2 = new Foo();
Console.WriteLine(foo1 == foo2);
// 5
IAutoEquatable<Foo> foo3 = new Foo();
IAutoEquatable<Foo> foo4 = new Foo();
Console.WriteLine(foo3 == foo4);

void Do<T>() where T :IAutoEquatable<T> , new()
{
    Console.WriteLine(new T() == new T());
}

void Do1<T>() where T : IEqualityOperators<T, T, bool> , new()
{
    Console.WriteLine(new T() == new T());
}

Expected behavior

1. ieop.== eq True
2. ieop.== eq True
3. ideally would be great to get ieop.== eq True but based on the fact that classes do not inherit members from its interfaces (default interface implementation spec) False can be somewhat explained
4. ieop.== eq True
5. ieop.== eq True

Actual behavior

1. ieop.== eq True
2. ieop.== eq True
3. False
4. False
5. False

Regression?

NA

Known Workarounds

NA

Configuration

• OS
  Windows 11
• SDK
  7.0.100 [C:\Program Files\dotnet\sdk]

Other information

@sharplab

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