Assertion failed with BuildFlavour = devel2

I tried to reproduce random ghc panic we have in our codebase with ghc8 so I decided to compile ghc with debug enabled.

Source file: ghc-8.0.1-src.tar.xz

build.mk changes:

BuildFlavour = devel2
GhcStage1HcOpts = -DDEBUG
GhcStage2HcOpts = -DDEBUG

source:

{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE PolyKinds #-}

module A where

import GHC.Generics

class B f where
  b :: [f a]
  
toEnumDefault :: (B (Rep a)) => Int -> a
toEnumDefault i = let l = b in to

result:

[1 of 1] Compiling A                ( a.hs, a.o )
ghc: panic! (the 'impossible' happened)
  (GHC version 8.0.1 for x86_64-unknown-linux):
        ASSERT failed!
  CallStack (from HasCallStack):
  assertPprPanic, called at compiler/types/TyCoRep.hs:1974:56 in ghc:TyCoRep
  checkValidSubst, called at compiler/types/TyCoRep.hs:2010:17 in ghc:TyCoRep
  substTy, called at compiler/types/TyCoRep.hs:1952:3 in ghc:TyCoRep
  in_scope InScope [a1BD :-> a_a1BD[sk], a1BI :-> k_a1BI[tau:3],
                    a1BJ :-> f_a1BJ[tau:3], a1BL :-> a_a1BL[sk]]
  tenv [a1BD :-> a_a1BL[sk]]
  tenvFVs [a1Bz :-> k_a1Bz[tau:5], a1BL :-> a_a1BL[sk]]
  cenv []
  cenvFVs []
  tys [[f_a1BJ[tau:3] a_a1BD[sk]]]
  cos []

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

Trac metadata

Trac field	Value
Version	8.0.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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