Many uses of conditional conformance to protocols with super-protocols
are for wrappers, where the conformances to those super-protocols
usually ends up using weaker bounds, such as:
struct MyWrapper<Wrapped: Sequence> {}
extension Wrapped: Sequence {} // Wrapped: Sequence
extension Wrapped: Collection where Wrapped: Collection {} // *
extension Wrapped: BidirectionalCollection where Wrapped: BidirectionalCollection {}
If this code was instead written:
struct MyWrapper<Wrapped: Sequence> {}
extension Wrapped: Sequence {} // Wrapped: Sequence
extension Wrapped: BidirectionalCollection where Wrapped: BidirectionalCollection {}
Inferring a Collection conformance would have to mean
extension Wrapped: Collection where Wrapped: BidirectionalCollection {}
which is unnecessarily strong.
It is a breaking change to change a protocol bound, and so we're
thinking we'd prefer that the compiler didn't magic up that incorrect
conformance. It also only is a small change (and the compiler even
suggests it, with a fixit) to explicitly get the implying behaviour:
declare the conformance explicitly.
extension Wrapped: Collection, BidirectionalCollection where Wrapped: BidirectionalCollection {}
Fixes rdar://problem/36499373.
