deriving Ix with custom ifThenElse causes "Bad call to tagToEnum#"

$ cat IfThenElseIx.hs
{-# LANGUAGE RebindableSyntax #-}
module IfThenElseIx where

import Data.Ix (Ix, )

import Prelude


ifThenElse :: Bool -> a -> a -> a
ifThenElse True x _ = x
ifThenElse False _ x = x

data T = A | B deriving (Eq, Ord, Ix)

$ ghci-8.0.0.20160109 IfThenElseIx.hs
GHCi, version 8.0.0.20160109: http://www.haskell.org/ghc/  :? for help
[1 of 1] Compiling IfThenElseIx     ( IfThenElseIx.hs, interpreted )

IfThenElseIx.hs:13:35: error:
    • Bad call to tagToEnum# at type a_a2ky
        Specify the type by giving a type signature
        e.g. (tagToEnum# x) :: Bool
    • In the second argument of ‘ifThenElse’, namely
        ‘(GHC.Prim.tagToEnum# (c# GHC.Prim.<=# b#))’
      In the expression:
        if (GHC.Prim.tagToEnum# (c# GHC.Prim.>=# a#)) then
            (GHC.Prim.tagToEnum# (c# GHC.Prim.<=# b#))
        else
            False
      In a case alternative:
          c#
            -> if (GHC.Prim.tagToEnum# (c# GHC.Prim.>=# a#)) then
                   (GHC.Prim.tagToEnum# (c# GHC.Prim.<=# b#))
               else
                   False
      When typechecking the code for ‘GHC.Arr.inRange’
        in a derived instance for ‘Ix T’:
        To see the code I am typechecking, use -ddump-deriv
Failed, modules loaded: none.

Trac metadata

Trac field	Value
Version	8.0.1-rc1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture

changed weight to 5

added Tbug Trac import labels

changed milestone to %8.0.1

changed weight to 10

Regression. Reproducible with HEAD.

Trac metadata

Trac field	Value
Priority	normal → highest

This is a regression relative to 7.10.3.

assigned to @bgamari

This comes from the inRange case generated for Ix. We decided to use guards instead.

Trac metadata

Trac field	Value
Differential revisions	- → D1797

mentioned in commit 5cce0954

closed

Merged to ghc-8.0 as 82efc3e6.

Trac metadata

Trac field	Value
Resolution	Unresolved → ResolvedFixed

mentioned in issue #12688 (closed)

added deriving label

added Phighest label