[stdlib] add missing Element type witnesses

[kavon]

With NoncopyableGenerics enabled, we currently lose some ability for associatedtype inference to find a suitable type witness based on a value witness. (rdar://118998138)

The stdlib accidentally uses that inference for Sequence.Element, due to the default witness for _customContainsEquatableElement mentioning Iterator.Element whereas the requirement only states Element.

[glessard]

Good find, thanks!

[glessard]

@swift-ci please test

[kavon]

I'm fine with making these typealiases present even when NoncopyableGenerics is disabled. It should be a non-functional change.

[slavapestov]

Can we revert this after fixing rdar://118998138? Doesn't this mean in its present state, NoncopyableGenerics could break non-stdlib code that hits this bug too?

[kavon]

@swift-ci please test macOS platform

[kavon]

Can we revert this after fixing rdar://118998138 (associatedtype inference fails to check conformances correctly)?

Yes, once that associatedtype inference issue is resolved, in theory we can revert this patch. But @glessard and others have expressed a desire to make these typealiases explicit anyway, so they're going to end up becoming non-conditional in another PR. I'm just adding these in now since I'm going to rely on them soon.

Doesn't this mean in its present state, NoncopyableGenerics could break non-stdlib code that hits this bug too?

Yes. Until rdar://118998138 is fixed non-stdlib code relying on that inference is broken.

[slavapestov]

But @glessard and others have expressed a desire to make these typealiases explicit anyway, so they're going to end up becoming non-conditional in another PR.

I think there's a good argument that this should not require an explicit type witness:

protocol P {
  associatedtype A: Q
  associatedtype B where B == A.B
}

protocol Q {
  associatedtype B
}

struct S: P {
  struct A: Q {
    typealias B = Int
  }

  // should not be needed:
  // typealias B = Int
}

[glessard]

It's fine to not require them, but the standard library can stand to be as explicit as possible.