RequirementMachine: Introduce 'concrete contraction' pre-processing pass before building rewrite system #41554
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…er superclass requirements again
If you have something like this:
protocol P {
associatedtype A : Q where Self == Self.A.B
}
protocol Q {
associatedtype B
}
class C : P {
typealias A = D
}
class D : P {
typealias B = C
}
The GSB would minimize the generic signature <T where T : P, T : C>
to <T where T == C>, because of the same-type requirement in
protocol P.
However in reality, the conformance 'C : P' is unsound, because it
is no longer covariant. I added a warning in commit d831eff74879cb.
Because the upcoming 'concrete contraction' pass eliminates 'T : P'
before homotopy reduction gets a chance to run, I'm changing the
Requirement Machine to produce a different minimization from the
GSB. This is technically an ABI break, but it should not impact any
real code in practice. If it does, I'll need to come up with a
different workaround in concrete contraction.
…o a non-conforming type The conflicting rule here is the permanent 'identity conformance' ([P].[P] => [P]). Permanent rules cannot be marked as conflicting, so just check for this condition first.
…ass before building rewrite system See the comment at the top of ConcreteContraction.cpp for a detailed explanation. This can be turned off with the -disable-requirement-machine-concrete-contraction pass, mostly meant for testing. A few tests now run with this pass both enabled and disabled, to exercise code paths which are otherwise trivially avoided by concrete contraction. Fixes rdar://problem/88135912.
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See the comment at the top of ConcreteContraction.cpp for a detailed explanation.
Fixes rdar://problem/88135912.