RequirementMachine: Fix minimization when protocol is constrained to a class that conforms to the protocol #41615
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…a class that conforms to the protocol
Consider this example:
protocol P : C {}
class C : P {}
<T where T : P>
The GenericSignatureBuilder thinks the minimized signature is
<T where T : P>. The RequirementMachine would minimize it down to
<T where T : C>. The latter is more correct, since the conformance
here is concrete and no witness table needs to be passed in at
runtime, however for strict binary compatibility we need to produce
the same signature as the GenericSignatureBuilder.
Accomplish this by changing the minimal conformances algorithm to
detect "circular concrete conformance rules", which take the form
[P].[concrete: C : Q]
Where Q : P. These rules are given special handling. Ordinarily a
protocol conformance rule is eliminated before a concrete conformance
rule; however concrete conformances derived from circular
conformances are considered to be redundant from the get-go,
preventing protocol conformances that can be written in terms of
such concrete conformances from themselves becoming redundant.
Fixes rdar://problem/89633532.
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Consider this example:
The GenericSignatureBuilder thinks the minimized signature is
<T where T : P>. The RequirementMachine would minimize it down to<T where T : C>. The latter is more correct, since the conformance here is concrete and no witness table needs to be passed in at runtime, however for strict binary compatibility we need to produce the same signature as the GenericSignatureBuilder.Accomplish this by changing the minimal conformances algorithm to detect "circular concrete conformance rules", which take the form
Where Q : P. These rules are given special handling. Ordinarily a protocol conformance rule is eliminated before a concrete conformance rule; however concrete conformances derived from circular conformances are considered to be redundant from the get-go, preventing protocol conformances that can be written in terms of such concrete conformances from themselves becoming redundant.
Fixes rdar://problem/89633532.