[GHC] #12826: TyVar ASSERT failure in type family checking: T12041

[GHC]

#12826: TyVar ASSERT failure in type family checking: T12041
-------------------------------------+-------------------------------------
           Reporter:  simonpj        |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 In HEAD, if you build the compiler with -DDEBUG, test `indexed-
 types/should_fail/T12041` fails thus
 {{{
 +ghc: panic! (the 'impossible' happened)
 +  (GHC version 8.1.20161109 for x86_64-unknown-linux):
 +       ASSERT failed!
 +  i_axb
 +  Call stack:
 +      CallStack (from HasCallStack):
 +        prettyCurrentCallStack, called at
 compiler/utils/Outputable.hs:<line>:<column> in <package-id>:Outputable
 +        callStackDoc, called at
 compiler/utils/Outputable.hs:<line>:<column> in <package-id>:Outputable
 +        assertPprPanic, called at
 compiler/typecheck/TcType.hs:<line>:<column> in <package-id>:TcType
 +  Call stack:
 +      CallStack (from HasCallStack):
 +        prettyCurrentCallStack, called at
 compiler/utils/Outputable.hs:<line>:<column> in <package-id>:Outputable
 +        callStackDoc, called at
 compiler/utils/Outputable.hs:<line>:<column> in <package-id>:Outputable
 +        pprPanic, called at compiler/utils/Outputable.hs:<line>:<column>
 in <package-id>:Outputable
 +        assertPprPanic, called at
 compiler/typecheck/TcType.hs:<line>:<column> in <package-id>:TcType
 }}}
 A `TyVar` is showing up where a `TcTyVar` is expected.

 I'm pretty certain this is harmless in a production compiler, but reflects
 an infelicity somewhere.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12826>
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