Restrict interface availability depending on parameter

[be5invis]

Objective: Let List<T> implement IEquatable<List<T>> if and only if T implements IEquatable<T>.
Like in Haskell:

instance (Eq a) => Eq [a] where ...

[casperOne]

I definitely don't want this. I'd rather have it in another type extending List<T> which has equality semantics.

The question of ordering and how it relates to equality isn't addressed here either.

[HaloFour]

The CLR has no facility to allow conditional interfaces at all, let alone driven by a generic type argument. How would you propose such a feature be implemented in CLR metadata?

[alrz]

Related: #164

extension class ListExtensions<T> : List<T>, IEquatable<List<T>> where T : IEquatable<T> { ... }

// alternative syntax, just for the sake of readability
implement<T> IEquatable<List<T>> for List<T> where T : IEquatable<T> { .. }

However, the proposal does not cover implementing regular interfaces (because it most likely requires clr support, along with "default interface methods", maybe), but I think that's definitely on the table.