swift

afa08f01 - GSB: Formalize the old hack where we rebuild a signature that had redundant conformance requirements

GSB: Formalize the old hack where we rebuild a signature that had redundant conformance requirements When constructing a generic signature, any redundant explicit requirements are dropped from the final signature. We would assume this operation is idempotent, that is, building a new GenericSignatureBuilder from the resulting minimized signature produces an equivalent GenericSignatureBuilder to the original one. Unfortunately, this is not true in the case of conformance requirements. Namely, if a conformance requirement is made redundant by a superclass or concrete same-type requirement, then dropping the conformance requirement changes the canonical type computation. For example, consider the following: public protocol P { associatedtype Element } public class C<O: P>: P { public typealias Element = O.Element } public func toe<T, O, E>(_: T, _: O, _: E, _: T.Element) where T : P, O : P, O.Element == T.Element, T : C<E> {} In the generic signature of toe(), the superclass requirement 'T : C<E>' implies the conformance requirement 'T : P' because C conforms to P. However, the presence of the conformance requirement makes it so that T.Element is the canonical representative, so previously this signature was minimized down to: <T : C<E>, O : P, T.Element == O.Element> If we build the signature again from the above requirements, then we see that T.Element is no longer the canonical representative; instead, T.Element canonicalizes as E.Element. For this reason, we must rebuild the signature to get the correct canonical type computation. I realized that this is not an artifact of incorrect design in the current GSB; my new rewrite system formalism would produce the same result. Rather, it is a subtle consequence of the specification of our minimization algorithm, and therefore it must be formalized in this manner. We used to sort-of do this with the HadAnyRedundantRequirements hack, but it was both overly broad (we only need to rebuild if a conformance requirement was implied by a superclass or concrete same-type requirement) and not sufficient (when rebuilding, we need to strip any bound associated types from our requirements to ensure the canonical type anchors are re-computed). Fixes rdar://problem/65263302, rdar://problem/75010156, rdar://problem/75171977.