[GHC] #11187: Odd interaction between type families and coercions (regression)

[GHC]

#11187: Odd interaction between type families and coercions (regression)
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           Reporter:  dfeuer         |             Owner:
               Type:  bug            |            Status:  new
           Priority:  high           |         Milestone:
          Component:  Compiler       |           Version:  7.10.2
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 {{{#!hs
 {-# LANGUAGE TypeFamilies #-}

 module Wonk where
 import Data.Type.Coercion

 type family X

 coercionXX :: Coercion X X
 coercionXX = Coercion
 }}}

 This works just fine in 7.8.3, but in 7.10.2 it gives the following
 peculiar error:

 {{{
     Couldn't match representation of type ‘X’ with that of ‘X’
     NB: ‘X’ is a type function, and may not be injective
     In the expression: Coercion
     In an equation for ‘coercionXX’: coercionXX = Coercion
 }}}

 I don't even know what it means for a ''nullary'' type function to be
 injective.
 I tried the following workarounds:

 {{{#!hs
 coercionXX1 :: Coercion X X
 coercionXX1 = c where
   c :: x ~ X => Coercion x x
   c = Coercion

 coercionXX2 :: Coercion X X
 coercionXX2 = c where c = Coercion
 }}}

 Interestingly, `coercionXX1` seems to work under all circumstances,
 with or without the top-level type signature; `coercionXX2` works
 if it's in a `.hs` file, but does ''not'' work if entered by hand
 at the `ghci` prompt (it produces a similar error).

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11187>
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