Default Interface Method Base Calls

[gafter]

The LDM decided on the base invocation syntax base(N.I1<T>).M(s). We also decided that the syntax should be permitted in classes (e.g. to invoke an implementation from a skip-base class or a base interface).

This needs to be implemented in the compiler. Also, we need to decide on an implementation strategy. What IL would that generate? Since interface implementations are currently private methods, it may require further CLR changes.

[gafter]

According to my notes, for a base call like

base(Type).M();

where Type is an interface type,

1. The invocation needs to be unambiguous - there needs to be a unique declaration of M that is binds to. That declaration would be in the type Type or some base interface of Type.
2. The compiler also needs to check that Type has a unique most specific override for the method selected by overload resolution. In other words, the following must be an error

interface A
{
    void M();
}
interface B : A
{
    override A.M() {}
}
interface C : A
{
    override A.M() {}
}
interface D : B, C
{
}
interface E : D
{
    override A.M()
    {
        base(D).M(); // error due to A.M not having a most specific override in D
    }
}

The marked line satisfies condition (1) as A.M is the only method to call, but fails condition (2).

[AlekseyTs]

LDM (2019-04-29) decided to remove the base(Type) syntax from the scope of C# 8.0. This issue is tracking the work to put it back into the product once we have support from CLR.

[AlekseyTs]

Closing for now because we don't have concrete plan to add this feature in the next release.

[AndriySvyryd]

Related: dotnet/csharplang#2337