Make use of boot TyThings during typechecking.
Summary: Suppose that you are typechecking A.hs, which transitively imports, via B.hs, A.hs-boot. When we poke on B.hs and discover that it has a reference to a type from A, what TyThing should we wire it up with? Clearly, if we have already typechecked A, we should use the most up-to-date TyThing: the one we freshly generated when we typechecked A. But what if we haven't typechecked it yet? For the longest time, GHC adopted the policy that this was *an error condition*; that you MUST NEVER poke on B.hs's reference to a thing defined in A.hs until A.hs has gotten around to checking this. However, actually ensuring this is the case has proven to be a bug farm. The problem was especially poignant with type family consistency checks, which eagerly happen before any typechecking takes place. This patch takes a different strategy: if we ever try to access an entity from A which doesn't exist, we just fall back on the definition of A from the hs-boot file. This means that you may end up with a mix of A.hs and A.hs-boot TyThings during the course of typechecking. Signed-off-by: Edward Z. Yang <email@example.com> Test Plan: validate Reviewers: simonpj, bgamari, austin, goldfire Subscribers: thomie, rwbarton GHC Trac Issues: #14396 Differential Revision: https://phabricator.haskell.org/D4154