AssociatedTypeInference: Delay substitutions into abstract type witnesses until after they have been computed

[AnthonyLatsis]

• We were trying to substitute into default type witnesses without considering other tentative abstract type witnesses we may need to compute later, resulting in conformance errors whenever a default type witness referenced other associated types that are destined to resolve to abstract type witnesses.

  protocol P {
    associatedtype A = [B]
    associatedtype B = Never
  }
  struct Foo: P {} // used to be a conformance error

• Bonus: fixed type witnesses are now getting substituted into as well and can finally reference other associated types without unconditional conformance errors.

  protocol P {
    associatedtype A = Never
    associatedtype B
  }
  protocol Q: P where B == [A] {}
  struct Foo: Q {} // used to be a conformance error

[AnthonyLatsis]

@swift-ci please smoke test

[AnthonyLatsis]

@swift-ci please test source compatibility release

[slavapestov]

Is this really necessary? Conformance lookup doesn't trigger computation of anything, it just returns an existing conformance even if its type witnesses have not yet been filled in.

[AnthonyLatsis]

This is a paranoia I have about conformance lookup determinism dating back to this comment by Joe :) (though I must admit the question is still open to me – how we could end up with a different conformance –, given that we cache them for as long as the nominal decl exists).

[AnthonyLatsis]

@swift-ci please test source compatibility release

[AnthonyLatsis]

@slavapestov LMK what you think of the additional modeling. Seems an exaggeration for the purpose of removing the mapIntoContext call, but I believe it has the potential of becoming beneficial to diagnostics and dumping later on.

[AnthonyLatsis]

@swift-ci please smoke test

[AnthonyLatsis]

@swift-ci please test source compatibility release