[GHC] #13981: Family instance consistency checks happens too early when hs-boot defined type occurs on LHS

[GHC]

#13981: Family instance consistency checks happens too early when hs-boot defined
type occurs on LHS
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        Reporter:  ezyang            |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  8.2.1-rc2
  checker)                           |
      Resolution:                    |             Keywords:  hs-boot
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by ezyang):

 Yeah. The details are in `Note [Don't check hs-boot type family instances
 too early]`, but reproduced below:

 * So, the origin of a panic "tcIfaceGlobal (local): not found" is when
 force the TyThing of a locally defined type prior to having actually
 typechecked that type. Ordinarily this can't happen (since you can't even
 get your hands on the TyThing before you've typechecked the type), but
 when you have an hs-boot file, a placeholder thunk for the TyThing is put
 into the context, and the onus is on you to avoid forcing this thunk until
 it is ready.

 * Type family consistency checking happens prior to any typechecking, and
 (as first discovered in #11062) it can cause a TyThing of an hs-boot
 defined type family to be forced too early. Originally, I suggested that
 we simply do the consistency check after all typechecking was done. But
 this has its own problem: if we do the consistency check too late, we may
 end up typechecking Haskell code under inconsistent/overlapping sets of
 axioms, which could lead to very strange errors.

 * Thus, we have to defer some of the consistency checks. We first do all
 of the consistency checks that could not possibly force a TyThing
 immediately, and then as we finish kind-checking a type family, we then go
 ahead and do all of the deferred consistency checks involving that family.

 * What I didn't realize when I originally wrote this patch is that the
 early forcing isn't limited just to a type family: types which occur in
 the LHS of overlapping instances (`T` above) also have this problem. So
 some new solution needs to be found.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13981#comment:2>
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