[GHC] #14326: Panic on COMPLETE pragma with mismatched type variable order

[GHC]

#14326: Panic on COMPLETE pragma with mismatched type variable order
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        Reporter:  dailectic         |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.2.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:  x86_64
 Type of failure:  Compile-time      |  (amd64)
  crash or panic                     |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Description changed by dailectic:

Old description:

> The motivating example is to allow patterns with phantom types to
> interact nicely with TypeApplications, so in the below we can do `L @Int
> :: a -> L a Int` to specify the phantom.
> {{{#!hs
> data E a b = L' a | R b
> pattern L :: forall b a. a -> E a b
> pattern L a = L' a
> {-# COMPLETE L, R #-}
> }}}
>
> The compiler balks
>
> {{{
> ghc: panic! (the 'impossible' happened)
>   (GHC version 8.2.1 for x86_64-unknown-linux):
>         mkOneConFull: Not TyConApp:
>   c_a50V
>   Call stack:
>       CallStack (from HasCallStack):
>         prettyCurrentCallStack, called at
> compiler/utils/Outputable.hs:1133:58 in ghc:Outputable
>         callStackDoc, called at compiler/utils/Outputable.hs:1137:37 in
> ghc:Outputable
>         pprPanic, called at compiler/deSugar/Check.hs:976:30 in ghc:Check
>
> Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug
>
> }}}

New description:

 The motivating example is to allow patterns with phantom types to interact
 nicely with TypeApplications, so in the below we can do `L @Int :: a -> L
 a Int` to specify the phantom.
 {{{#!hs
 data E a b = L' a | R b
 pattern L :: forall b a. a -> E a b
 pattern L a = L' a
 {-# COMPLETE L, R #-}
 }}}

 Issues occur when nesting cases

 {{{
 {-# OPTIONS_GHC -fwarn-incomplete-patterns #-}
 testMono :: E (E Int Int) Int -> Int
 testMono x = case x of
   L (L _) -> 0
   L (R _) -> 1
   R _ -> 2
 }}}

 And GHC panics when polymorphic

 {{{
 {-# OPTIONS_GHC -fwarn-incomplete-patterns #-}
 testPoly :: E (E a b) c -> Int
 testPoly x = case x of
   L (L _) -> 0
   L (R _) -> 1
   R _ -> 2
 }}}

 The compiler balks

 {{{
 ghc: panic! (the 'impossible' happened)
   (GHC version 8.2.1 for x86_64-unknown-linux):
         mkOneConFull: Not TyConApp:
   c_a50V
   Call stack:
       CallStack (from HasCallStack):
         prettyCurrentCallStack, called at
 compiler/utils/Outputable.hs:1133:58 in ghc:Outputable
         callStackDoc, called at compiler/utils/Outputable.hs:1137:37 in
 ghc:Outputable
         pprPanic, called at compiler/deSugar/Check.hs:976:30 in ghc:Check

 Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug

 }}}

 So it seems like there's two related issues here:
 1. The COMPLETE pragma does not work in nesting for out-of-order type
 variables
 2. Some printing logic is missing for the error in this polymorphic case

 Note that if we define

 {{{
 pattern L :: forall a b. a -> E a b
 pattern L a = L' a
 }}}

 Note that we do not get the incomplete warning or the panic, so it seems
 directly related to the order of the variables, rather than explicit
 quantification in general

--

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14326#comment:2>
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