[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
      Resolution:                    |             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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Comment (by dbeacham):

 Here's a much shorter example without TH or Vector.

 {{{
 {-# LANGUAGE TypeFamilies #-}

 {-# LANGUAGE StrictData #-}
 {-# OPTIONS_GHC -funbox-strict-fields #-}

 module Lib where

 data V a = V a a
   deriving (Show)

 data family Test a

 newtype instance Test (V a) = V_V (a, a)
 }}}

 with this error:

 {{{
 ghc: panic! (the 'impossible' happened)
   (GHC version 8.6.3 for x86_64-unknown-linux):
         applyTypeToArgs
   Expression: V_V @ a_a1BO dt_a1R2 dt_a1R3
   Type: forall a. (a, a) -> Test (V a)
   Args: [TYPE: a_a1BO, dt_a1R2, dt_a1R3]
   Call stack:
       CallStack (from HasCallStack):
         callStackDoc, called at compiler/utils/Outputable.hs:1160:37 in
 ghc:Outputable
         pprPanic, called at compiler/coreSyn/CoreUtils.hs:234:14 in
 ghc:CoreUtils
 }}}

 Removing either StrictData or -funbox-strict-fields makes it compile.

 Maybe of interest that removing StrictData but adding in explicit
 strictness compiles OK:

 {{{
 {-# LANGUAGE TypeFamilies #-}

 {-# OPTIONS_GHC -funbox-strict-fields #-}

 module Lib where

 data V a = V !a !a
   deriving (Show)

 data family Test a

 newtype instance Test (V a) = V_V (a, a)

 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/16191#comment:3>
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