[GHC] #11648: assertPprPanic, called at compiler/types/TyCoRep.hs:1932

[GHC]

#11648: assertPprPanic, called at compiler/types/TyCoRep.hs:1932
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        Reporter:  thomie            |                Owner:  goldfire
            Type:  bug               |               Status:  new
        Priority:  highest           |            Milestone:  8.0.1
       Component:  Compiler          |              Version:  8.1
      Resolution:                    |             Keywords:  TypeInType
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 simonpj):

 > I'm not sure what you mean by "quite different"

 Suppose you instantiate `f :: forall k. forall (a :: kappa). blah` at some
 call site. Make up a fresh meta-var for `k` and substitute.  We won't
 substitute for `kappa` because it isn't `k`.

 Later we unify `kappa := k`.  And now our earlier instantiation of `f`'s
 type is utterly wrong.

 Actually the real INVARIANT is probably this:

 * In any quantified type `forall a. blah`, all occurrences of `a` in
 `blah` must be manifest: not hidden inside meta-tyvars, especially if they
 have not been instantiated.

 There could in principle be some free meta-tyvars that we somehow know can
 never be filled in with the quantified type variable.  But for top-level
 types or kinds this is never necessary.

 Anyway I agree with your plan.  But

 > But I don't spot a missing `solveEqualities` anywhere. Did you miss the
 call toward the top of `kcTyClGroup`?

 I'm still looking at `kcHsTyVarBndrs`. It builds a polytype with
 `mkSpecForAllTys`.  It does unification (inside the call to
 `tcHsTyVarBndr_Scoped`). I think we need to solve the equalities before we
 bind the type variables that are mentioned, don't we?  Ugh -- that
 `kcHsTyVarBndrs` is horribly complicated.

 We should probably Skype but I'm out all Thurs.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11648#comment:19>
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