If a conformance to a protocol is implied by several other
conformances (i.e. protocol P: Equatable {} and protocol Q: Equatable {} and a
type declares conformance to both P and Q), we should choose a source that's in
the same file as the type, if we can, because automatic synthesis of
conformances (for Equatable, Hashable, etc.) only works in that case.
Fixes rdar://problem/41852654.
'private' properties can't be accessed in extensions in Swift 3, so synthesizing
a conformance that reads from such things is going to be incorrect in an
extension.
This works for all protocols except for Decodable on non-final classes, because
the init requirement has to be 'required' and thus in the type's declaration.
Fixes most of https://bugs.swift.org/browse/SR-6803.
Implements the minimum specified by the SE-proposal.
* Add the CaseIterable protocol with AllCases associatedtype and
allCases requirement
* Automatic synthesis occurs for "simple" enums
- Caveat: Availability attributes suppress synthesis. This can be
lifted in the future
- Caveat: Conformance must be stated on the original type
declaration (just like synthesizing Equatable/Hashable)
- Caveat: Synthesis generates an [T]. A more efficient collection
- possibly even a lazy one - should be put here.
