Type families + hs-boot files = unsafeCoerce

Consider the following bundle of modules:

A.hs:

{-# LANGUAGE TypeFamilies #-}

module A where

type family F a b

B.hs-boot:

module B where

import A

oops :: F a b -> a -> b

B.hs:

{-# LANGUAGE TypeFamilies #-}

module B where

import A
import C

type instance F a b = b

oops :: F a b -> a -> b
oops = const

C.hs:

module C (oops) where

import {-# SOURCE #-} B

D.hs:

{-# LANGUAGE TypeFamilies #-}

module D where

import A
import C

type instance F a b = a

unsafeCoerce :: a -> b
unsafeCoerce x = oops x x

Main.hs:

module Main where

import D ( unsafeCoerce )

main = print $ (unsafeCoerce True :: Int)

When loading these into GHCi, we quite reasonably get a type family instance overlap error. But, separate compilation leads to disaster:

rae:01:49:47 ~/temp/bug> ghc --version
The Glorious Glasgow Haskell Compilation System, version 7.8.3
rae:01:49:49 ~/temp/bug> ghc -c A.hs
rae:01:49:53 ~/temp/bug> ghc -c B.hs-boot 
rae:01:49:58 ~/temp/bug> ghc -c C.hs
rae:01:50:09 ~/temp/bug> ghc -c B.hs
rae:01:50:13 ~/temp/bug> ghc -c D.hs
rae:01:50:17 ~/temp/bug> ghc Main.hs -o Unsafe
[6 of 6] Compiling Main             ( Main.hs, Main.o )
Linking Unsafe ...
rae:01:50:23 ~/temp/bug> ./Unsafe
2882303761534249061

Yikes!

Proposed (terrible) solution: hs-boot files must list all type instance declarations in the corresponding modules. It may also be a good idea to require all normal instance declarations in the hs-boot file as well, because this same trick can be used to introduce incoherence (I think -- haven't tested).

This bug persists even if Main declares that it is Safe.

I've attached a tarball of the files for ease of testing.

(Credit to Edward Yang and Geoff Mainland, whose discussion provoked the line of inquiry that led to this discovery.)

Trac metadata

Trac field	Value
Version	7.8.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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