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

[GHC]

#11648: assertPprPanic, called at compiler/types/TyCoRep.hs:1932
-------------------------------------+-------------------------------------
        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:                    |
-------------------------------------+-------------------------------------

Comment (by goldfire):

 Ah -- so the meta-tyvars aren't making it to the real `TyCon`, only the
 `TcTyCon`? It's quite normal for `TcTyCon`s to have meta-tyvars. (Indeed,
 that's what they're for.)

 But I'm missing some context. The ASSERT that's breaking is the check
 during substitutions. But which substitution? I don't see an obvious
 substitution in `getFamDeclInitialKind`.

 To be clear, I think there are two related problems going on:
  1. The CUSK check is inaccurate in the presence of `TypeInType`, because
 implicitly-bound kind variables (like `k` in `data Proxy (a :: k)`) can
 have non-trivial types.
  2. The code in !TcTyClsDecls is tripping over CUSK types that aren't
 actually CUSKs.

 I propose to fully defer (1). It can be triggered only with `TypeInType`
 enabled and has some thorny design issues. And when it goes wrong, it
 won't be catastrophic (i.e., no panicking, segfaulting, or launching
 rockets, just an unexpected -- but still sound -- result of type
 inference).

 (2) is the real issue at hand. And, currently without the ability to test,
 I don't fully understand what's going wrong. I hope to be out of my
 intermediate state today or tomorrow.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11648#comment:14>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler