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.
Recently I changed the ArchetypeBuilder is minimize requirements
in generic signatures. However substitution lists still contained
all recursively-expanded nested types.
With recursive conformances, this list becomes potentially
infinite, so we can't expand it out anymore. Also, it is just
a waste of time to have them there.
The existing code assumed that the _bridgeToObjectiveC witness would never come from a protocol extension. Generating the correct set of substitutions is a bit harder than it ought to be, due to inconsistencies in how Sema represents generic witnesses. Fixes rdar://problem/28279269.
