Allow inref-argument members to satisfy SRTP constraints

Transferring https://github.com/dotnet/fsharp/issues/9018

In C#, you can mark the parameters of an overloaded operator as inref:

public readonly struct C
{
    public readonly int I {get;}
    public C(int i)
    {
        I = i;
    }

    public void M()
    {
        var c1 = new C(1);
        var c2 = new C(2);

        var c3 = c1 + c2;
    }

    public static C operator+ (in C a, in C b)
    {
        return new C(a.I + b.I);
    }
}

This operator is not directly usable in F#:

    let c1 = C(1)
    let c2 = C(1)
    let c3 = c1 + c2;

error FS0001: Type constraint mismatch. The type 'C' is not compatible with type 'inref' error FS0043: Type constraint mismatch. The type 'C' is not compatible with type 'inref'

As a workaround, you can explicitly call op_Addition:

let c3 = C.op_Addition(&c1, &c2)

For the first part, this is by design as things currently stand.

a + b is not a method call but a call to the F# library operator for (+) with its SRTP constraint
the inref version of the op_Addition member provided by C# does not satisfy this SRTP constraint

We could consider a language suggestion where members taking inref arguments can be considered to satisfy such constraints but it would need an RFC. I'm not sure of its technical feasiblity.

fsharp / fslang-suggestions

Allow inref-argument members to satisfy SRTP constraints #912