[GHC] #16251: T5515 broken in devel2 flavour

[GHC]

#16251: T5515 broken in devel2 flavour
-------------------------------------+-------------------------------------
           Reporter:  bgamari        |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.6.3
           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:
-------------------------------------+-------------------------------------
 The `T5515` testcase is broken in the `devel2` build flavour:
 {{{
 --- indexed-types/should_fail/T5515.run/T5515.stderr.normalised 2019-01-27
 16:01:12.703091135 +0000
 +++ indexed-types/should_fail/T5515.run/T5515.comp.stderr.normalised
 2019-01-27 16:01:12.703091135 +0000
 @@ -1,24 +1,12 @@
 +ghc: panic! (the 'impossible' happened)
 +  (GHC version 8.7.20190127 for x86_64-unknown-linux):
 +       ASSERT failed!
 +  rhs axiom D:R:Argf ::
 +        Arg f = a_asO[sk:1] -- Defined at T5515.hs:15:8
 +  Call stack:
 +      CallStack (from HasCallStack):
 +        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/FamInst.hs:<line>:<column> in <package-id>:FamInst

 -T5515.hs:6:16:
 -     Expecting one more argument to ‘ctx’
 -      Expected a type, but ‘ctx’ has kind ‘* -> Constraint’
 -     In the first argument of ‘Arg’, namely ‘ctx’
 -      In the first argument of ‘ctx’, namely ‘(Arg ctx)’
 -      In the class declaration for ‘Bome’
 -
 -T5515.hs:14:1:
 -     Type variable ‘a’ is mentioned in the RHS,
 -        but not bound on the LHS of the family instance
 -     In the type instance declaration for ‘Arg’
 -      In the instance declaration for ‘Some f’
 -
 -T5515.hs:14:10:
 -     Could not deduce (C f a0)
 -      from the context: C f a
 -        bound by an instance declaration:
 -                   forall f a. C f a => Some f
 -        at T5515.hs:14:10-24
 -      The type variable ‘a0’ is ambiguous
 -     In the ambiguity check for an instance declaration
 -      To defer the ambiguity check to use sites, enable
 AllowAmbiguousTypes
 -      In the instance declaration for ‘Some f’
 +Please report this as a GHC bug:  https://www.haskell.org/ghc/reportabug
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/16251>
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