[GHC] #16191: GHC Panic deriving unbox vector (StrictData/unbox-strict-fields interaction)

[GHC]

#16191: GHC Panic deriving unbox vector (StrictData/unbox-strict-fields
interaction)
-------------------------------------+-------------------------------------
           Reporter:  dbeacham       |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.6.3
           Keywords:                 |  Operating System:  Linux
       Architecture:                 |   Type of failure:  GHC rejects
  Unknown/Multiple                   |  valid program
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 The attached program fails to compile with the following error:

 {{{
 ghc: panic! (the 'impossible' happened)
   (GHC version 8.6.3 for x86_64-unknown-linux):
         mkNewTyConRhs
   MV_V [Int#, MVector s_a7WC[sk:1] a_a7WA[sk:1],
         MVector s_a7WC[sk:1] a_a7WA[sk:1]]
   Call stack:
       CallStack (from HasCallStack):
         callStackDoc, called at compiler/utils/Outputable.hs:1160:37 in
 ghc:Outputable
         pprPanic, called at compiler/iface/BuildTyCl.hs:65:27 in
 ghc:BuildTyCl
 }}}

 Removing either of the -funbox-strict-fields or StrictData pragma allow
 the code to compile correctly, but you do need to compile with -O to get
 the incorrect behaviour -ghci is fine loading the module.

 Although I wouldn't expect them to interact here as I don't get the same
 behaviour if I explicitly write out what would be implied by the flag and
 pragma:

 {{{
 data V a = V {-# UNPACK #-} !a {-# UNPACK #-} !a
   deriving (Show)
 }}}

 just warnings about the unused UNPACKs.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/16191>
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