[GHC] #14174: GHC panic with TypeInType and type family

[GHC]

#14174: GHC panic with TypeInType and type family
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        Reporter:  dfeuer            |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.2.3
       Component:  Compiler (Type    |              Version:  8.2.1
  checker)                           |
      Resolution:                    |             Keywords:  TypeInType
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  Compile-time      |            Test Case:
  crash or panic                     |  polykinds/T14174.hs, T14174a
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by Simon Peyton Jones <simonpj@…>):

 In [changeset:"0a12d92a8f65d374f9317af2759af2b46267ad5c/ghc"
 0a12d92a/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="0a12d92a8f65d374f9317af2759af2b46267ad5c"
 Further improvements to well-kinded types

 The typechecker has the invariant that every type should be well-kinded
 as it stands, without zonking.  See Note [The well-kinded type invariant]
 in TcType.

 That invariant was not being upheld, which led to Trac #14174.  I fixed
 part of it, but T14174a showed that there was more.  This patch finishes
 the job.

 * See Note [The tcType invariant] in TcHsType, which articulates an
   invariant that was very nearly, but not quite, true.  One place that
   falisified it was the HsWildCardTy case of tc_hs_type, so I fixed that.

 * mkNakedCastTy now makes no attempt to eliminate casts; indeed it cannot
   lest it break Note [The well-kinded type invariant].  The prior comment
   suggested that it was crucial for performance but happily it seems not
   to be. The extra Refls are eliminated by the zonker.

 * I found I could tidy up TcHsType.instantiateTyN and instantiateTyUntilN
   by eliminating one of its parameters.  That led to a cascade of minor
   improvements in TcTyClsDecls. Hooray.
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14174#comment:8>
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