swift

4dc0db31 - Sema: Improve the 'protocol method can't impose new requirements on Self' check

Sema: Improve the 'protocol method can't impose new requirements on Self' check This check had two problems. First, it would assert upon encountering a layout requirement, due to an unimplemented code path. A more fundamental issue is that the logic wasn't fully sound, because it would miss certain cases, for example: protocol P { associatedtype A func run<B: Equatable>(_: B) where B == Self.A } Here, the reduced type of `Self.A` is `B`, and at first glance, the requirement `B: Equatable` appears to be fine. However, this is actually a new requirement on `Self`, and the protocol be rejected. Now that we can change the reduction order by assigning weights to generic parameters, this check can be implemented in a better way, by building a new generic signature first, where all generic parameters introduced by the protocol method, like 'B' above, are assigned a non-zero weight. With this reduction order, any type that is equivalent to a member type of `Self` will have a reduced type rooted in `Self`, at which point the previous syntactic check becomes sound. Since this may cause us to reject code we accepted previously, the type checker now performs the check once: first on the original signature, which may miss certain cases, and then again on the new signature built with the weighted reduction order. If the first check fails, we diagnose an error. If the second check fails, we only diagnose a warning. However, this warning will become an error soon, and it really can cause compiler crashes and miscompiles to have a malformed protocol like this. Fixes rdar://116938972.