`deriving instance Generic (HsExpr Id)` raises panic

[Daniel Schuessler]

Input file:

{-# LANGUAGE FlexibleInstances, DeriveGeneric, StandaloneDeriving #-}
module GHCGeneric where

import GHC
import GHC.Generics

deriving instance Generic (HsExpr Id)

Result:

GHCi, version 7.4.2: http://www.haskell.org/ghc/  :? for help
Loading package ghc-prim ... linking ... done.
Loading package integer-gmp ... linking ... done.
Loading package base ... linking ... done.
[1 of 1] Compiling GHCGeneric       ( /home/daniel/dev/dataflow/GHCGeneric.hs, interpreted )
ghc: panic! (the 'impossible' happened)
  (GHC version 7.4.2 for i386-unknown-linux):
        tcTyVarDetails ( id{tv a3cX} [tv] :: *{(w) tc 34d} )

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

Regards,
Daniel

[simonpj]

With HEAD we get

~/tmp$ ~/5builds/HEAD-1/inplace/bin/ghc-stage2 -c T7035.hs -package ghc

T7035.hs:8:1:
    Can't make a derived instance of `Generic (HsExpr Id)':
      HsExpr must not be instantiated; try deriving `HsExpr Id' instead
    In the stand-alone deriving instance for `Generic (HsExpr Id)'

Better than a crash, but the error message is messed up. Should say something like try deriving (HsExpr a) instead, or something. Currently it says "X doesn't work, so try X" which is stupid.

Pedro, can you look at this?

Simon

[dreixel]

Somehow I've only seen this now. I think it's fixed by now, but I'll check.

[dreixel]

Already fixed in #7255.