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

5 Oct 2017

      #14326: Panic on COMPLETE pragma with mismatched type variable order
-------------------------------------+-------------------------------------
        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:                    |
-------------------------------------+-------------------------------------
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 #-}
}}}
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
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
 {-# LANGUAGE PatternSynonyms #-}
 {-# LANGUAGE ExplicitForAll #-}
 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:3
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