GHCi panic in getIdFromTrivialExpr with -fdefer-type-errors

Steps to reproduce:

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

{-# 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

>

Trac metadata

Trac field	Value
Version	8.4.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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