Slava Pestov 64f12ff936 Sema: Allow protocols with 'Self' constraints again ...

These two declarations are now equivalent:

  protocol P : SomeClass { ... }
  protocol P where Self : SomeClass { ... }

There's a long, complicated story here:

- Swift 4.2 rejected classes in the inheritance clause of a
  protocol, but it accepted the 'where' clause form, even
  though it didn't always work and would sometimes crash

- Recently we got the inheritance clause form working, and
  added a diagnostic to ban the 'where' clause form, because
  we thought it would simplify name lookup to not have to
  consider the 'where' clause

- However, we already had to support looking at the 'where'
  clause from name lookup anyway, because you could write

  extension P where Self : SomeClass { ... }

- It turns out that despite the crashes, protocols with
  'Self' constraints were already common enough that it was
  worth supporting the existing behavior, instead of banning
  it

Fixes <rdar://problem/43028442>.

2018-10-12 03:06:52 -07:00

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