RebindableSyntax breaks deriving Ord

The code generated with deriving (Ord) includes if-expressions which are subject to rebindable syntax:

{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE RebindableSyntax #-}

import Prelude

class IfThenElse a b where
  ifThenElse :: a -> b -> b -> b

instance IfThenElse Bool b where
  ifThenElse c x y = if c then x else y

data Foo = Foo | Bar | Baz deriving (Eq, Ord)

main :: IO ()
main = print $ Foo < Bar

when loaded into ghci (7.10, but not specific to that version):

[1 of 1] Compiling Main             ( Foo.hs, interpreted )

Foo.hs:13:42:
    Bad call to tagToEnum# at type a_aQy
      Specify the type by giving a type signature
      e.g. (tagToEnum# x) :: Bool
    In the expression: (GHC.Prim.tagToEnum# (a# GHC.Prim.<# b#))
    In the expression:
      if (GHC.Prim.tagToEnum# (a# GHC.Prim.<# b#)) then
          LT
      else
          if (GHC.Prim.tagToEnum# (a# GHC.Prim.==# b#)) then EQ else GT
    In a case alternative:
        b#
          -> if (GHC.Prim.tagToEnum# (a# GHC.Prim.<# b#)) then
                 LT
             else
                 if (GHC.Prim.tagToEnum# (a# GHC.Prim.==# b#)) then EQ else GT
    When typechecking the code for  ‘compare’
      in a derived instance for ‘Ord Foo’:
      To see the code I am typechecking, use -ddump-deriv

Foo.hs:13:42:
    No instance for (IfThenElse a0 Ordering)
      arising from an if statement
    The type variable ‘a0’ is ambiguous
    Note: there is a potential instance available:
      instance IfThenElse Bool b -- Defined at Foo.hs:10:10
    In the expression:
      if (GHC.Prim.tagToEnum# (a# GHC.Prim.<# b#)) then
          LT
      else
          if (GHC.Prim.tagToEnum# (a# GHC.Prim.==# b#)) then EQ else GT
    In a case alternative:
        b#
          -> if (GHC.Prim.tagToEnum# (a# GHC.Prim.<# b#)) then
                 LT
             else
                 if (GHC.Prim.tagToEnum# (a# GHC.Prim.==# b#)) then EQ else GT
    In the expression:
      case (Main.$con2tag_rjG b) of {
        b#
          -> if (GHC.Prim.tagToEnum# (a# GHC.Prim.<# b#)) then
                 LT
             else
                 if (GHC.Prim.tagToEnum# (a# GHC.Prim.==# b#)) then EQ else GT }
    When typechecking the code for  ‘compare’
      in a derived instance for ‘Ord Foo’:
      To see the code I am typechecking, use -ddump-deriv
Failed, modules loaded: none.

The if-expressions are generated by nlHsIf, which calls mkHsIf, which uses Just noSyntaxExpr as its first argument. I'm going to add a new function nlBuiltInHsIf which uses Nothing as the first argument, forcing the built-in if (according to Note [Rebindable if]), then use this new function when deriving stuff.

I'll post a patch soonish.

Trac metadata

Trac field	Value
Version	7.10.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC	simonmar
Operating system
Architecture