[GHC] #15325: Panic in getIdFromTrivialExpr with -fdefer-type-errors

30 Jun 2018

      #15325: Panic in getIdFromTrivialExpr with -fdefer-type-errors
-------------------------------------+-------------------------------------
           Reporter:  dramforever    |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:  8.6.1
          Component:  Compiler       |           Version:  8.4.3
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  Compile-time
  Unknown/Multiple                   |  crash or panic
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 == Steps to reproduce:

 Put this in `bug.hs`: (It's a very failed attempt to make a polymorphic
 list maker function.)

 {{{#!hs
 {-# LANGUAGE FlexibleInstances #-}
 {-# LANGUAGE MultiParamTypeClasses #-}
 {-# LANGUAGE TypeFamilies #-}

 module PolyvariadicFunctions where

 class PolyList e a where
     polyList :: ([e] -> [e]) -> a

 -- instance PolyList e [e] where
 --     polyList dl = dl []

 instance (e ~ e2, PolyList e a) => PolyList e (e2 -> a) where
     polyList dl x = polyList ((x :) . dl)

 plh :: [Integer]
 plh = polyList 1 2 3 4 5
 }}}

 Load it in GHCi with `-fdefer-type-errors` to get the following output.
 Note the 'panic' in the end, and also note that the last `>` line is a no-
 module-loaded GHCi prompt.

 The question marks `?` seem to be encoding related and not relevant.

 This panic not seem to occur with GHC the compiler. The rest of the error
 messages seem otherwise identical.

 {{{
 GHCi, version 8.4.3: http://www.haskell.org/ghc/  :? for help
 Prelude> :set -fdefer-type-errors
 Prelude> :l bug.hs
 [1 of 1] Compiling PolyvariadicFunctions ( bug.hs, interpreted )

 bug.hs:17:7: warning: [-Wdeferred-type-errors]
     ? No instance for (PolyList t0 [Integer])
         arising from a use of ‘polyList’
     ? In the expression: polyList 1 2 3 4 5
       In an equation for ‘plh’: plh = polyList 1 2 3 4 5
    |
 17 | plh = polyList 1 2 3 4 5
    |       ^^^^^^^^^^^^^^^^^^

 bug.hs:17:16: warning: [-Wdeferred-type-errors]
     ? No instance for (Num ([t0] -> [t0])) arising from the literal ‘1’
         (maybe you haven't applied a function to enough arguments?)
     ? In the first argument of ‘polyList’, namely ‘1’
       In the expression: polyList 1 2 3 4 5
       In an equation for ‘plh’: plh = polyList 1 2 3 4 5
    |
 17 | plh = polyList 1 2 3 4 5
    |                ^

 bug.hs:17:18: warning: [-Wdeferred-type-errors]
     ? Ambiguous type variable ‘t0’ arising from the literal ‘2’
       prevents the constraint ‘(Num t0)’ from being solved.
       Probable fix: use a type annotation to specify what ‘t0’ should be.
       These potential instances exist:
         instance Num Integer -- Defined in ‘GHC.Num’
         instance Num Double -- Defined in ‘GHC.Float’
         instance Num Float -- Defined in ‘GHC.Float’
         ...plus two others
         (use -fprint-potential-instances to see them all)
     ? In the second argument of ‘polyList’, namely ‘2’
       In the expression: polyList 1 2 3 4 5
       In an equation for ‘plh’: plh = polyList 1 2 3 4 5
    |
 17 | plh = polyList 1 2 3 4 5
    |                  ^
 ghc.exe: panic! (the 'impossible' happened)
   (GHC version 8.4.3 for x86_64-unknown-mingw32):
         getIdFromTrivialExpr
   case $dNum_a1Be of wild_00 { }
   Call stack:
       CallStack (from HasCallStack):
         callStackDoc, called at compiler\utils\Outputable.hs:1150:37 in
 ghc:Outputable
         pprPanic, called at compiler\\coreSyn\\CoreUtils.hs:883:18 in
 ghc:CoreUtils

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

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15325
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