Incoherent instances without -XIncoherentInstances

Consider the following 4 modules:

{-# LANGUAGE GADTs #-}

module A where

data One = One

data ShowInstFor a = Show a => MkSIF

incoherent :: ShowInstFor One -> ShowInstFor One -> String
incoherent MkSIF MkSIF = show One

module B where

import A

instance Show One where
  show _ = "from module B"

fromB :: ShowInstFor One
fromB = MkSIF

module C where

import A

instance Show One where
  show _ = "from module C"

fromC :: ShowInstFor One
fromC = MkSIF

module D where

import A
import B
import C

oops1 = incoherent fromB fromC
oops2 = incoherent fromC fromB

According to ghci, oops1 is "from moduleB" and oops2 is "from module C". This seems bad.

Trac metadata

Trac field	Value
Version	7.6.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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