Doug Gregor 6e6b6b9837 [Mangler] Only mangle new requirements for constrained extensions. ...

Rather than mangling the complete generic signature of a constrained
extension, only mangle the requirements not already satisfied by the
nominal type. For example, given:

    extension Dictionary where Value: Equatable {
      // OLD: _T0s10DictionaryV2t3s8HashableRzs9EquatableR_r0_lE3baryyF
      // NEW: _T0s10DictionaryV2t3s9EquatableR_rlE3baryyF
      public func bar() { }
   }

In the existing mangling, we mangle the `Key: Hashable` requirement that’s
part of the generic signature. With this change, we only mangle the new
requirement (`Value: Equatable`).

This is a win for constrained extensions *except* in the case of a
constrained extension of a nominal type with a single, unconstrained
generic parameter:

    extension Array where Element: Equatable {
      // OLD: _T0Sa2t3s9EquatableRzlE3baryyF
      // NEW would be: _T0Sa2t3s9EquatableRzrlE3baryyF
      public func bar() { }
    }

Check explicily for this shortcut mangling and fall back to the old
path, so this change is a strict improvement.

2017-10-11 19:23:58 -07:00

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