Doug Gregor 99daad0f30 Rework witness matching for generic requirements. ...

Reimplement the witness matching logic used for generic requirements
so that it properly models the expectations required of the witness,
then captures the results in the AST. The new approach has a number of
advantages over the existing hacks:

* The constraint solver no longer requires hacks to try to tangle
  together the innermost archetypes from the requirement with the
  outer archetypes of the context of the protocol
  conformance. Instead, we create a synthetic set of archetypes that
  describes the requirement as it should be matched against
  witnesses. This eliminates the infamous 'SelfTypeVar' hack.
* The type checker no longer records substitutions involving a weird
  mix of archetypes from different contexts (see above), so it's
  actually plausible to reason about the substitutions of a witness. A
  new `Witness` class contains the declaration, substitutions, and all
  other information required to interpret the witness.
* SILGen now uses the substitution information for witnesses when
  building witness thunks, rather than computing all of it from
  scratch. ``substSelfTypeIntoProtocolRequirementType()` is now gone
  (absorbed into the type checker, and improved from there), and the
  witness-thunk emission code is simpler. A few other bits of SILGen
  got simpler because the substitutions can now be trusted.
* Witness matching and thunk generation involving generic requirements
  and nested generics now works, based on some work @slavapestov was
  already doing in this area.
* The AST verifier can now verify the archetypes that occur in witness substitutions.
* Although it's not in this commit, the `Witness` structure is
  suitable for complete (de-)serialization, unlike the weird mix of
  archetypes previously present.

Fixes rdar://problem/24079818 and cleans up an area that's been messy
and poorly understood for a very, very long time.

2016-10-30 23:15:43 -07:00

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