[CSSimplify] Delay binding metatype instance types if one side is type variable

[xedin]

While matching two metatypes where one side is a class (which could have
supertypes) and another is a type variable use subtype constraint to
delay binding types together because there is a real possibility that
there exists a superclass associated with yet free type variable which
would be a better match when attempted.

Resolves: rdar://problem/44816848

[xedin]

@swift-ci please test source compatibility

[xedin]

@swift-ci please test

[swift-ci]

Build failed
Swift Test OS X Platform
Git Sha - 62d728c626a90de54d79263888ffc702eb3a75a6

[xedin]

Timeout :(

[xedin]

@swift-ci please test macOS platform

[rudkx]

The metatype handling code here (prior to your change) seems pretty odd to me. Why do we default to a type equality check regardless of what the original type match kind was?

I wonder what breaks if we just make this immediately call back into matchTypes with the same kind, and the instance types?

[xedin]

@rudkx I think the reason is trying to maintain subtype property? I wonder if what we need to to do here is to resolve this constraint (e.g. call to matchTypes) only if both types are known, otherwise just form unresolved, because currently e.g. in case of ExistentialMetatype only one side is checked...

[xedin]

@swift-ci please test

[swift-ci]

Build failed
Swift Test Linux Platform
Git Sha - 62d728c626a90de54d79263888ffc702eb3a75a6

[swift-ci]

Build failed
Swift Test OS X Platform
Git Sha - 62d728c626a90de54d79263888ffc702eb3a75a6

[rudkx]

Change LGTM. Might be good to follow-up at some point with an assert about the incoming kind so that if we ever violate any assumptions being made here it will be flagged.

[xedin]

@swift-ci please test source compatibility