[GHC] #10009: type inference regression when faking injective type families

[GHC]

#10009: type inference regression when faking injective type families
-------------------------------------+-------------------------------------
        Reporter:  aavogt            |                   Owner:
            Type:  bug               |                  Status:  new
        Priority:  high              |               Milestone:  7.10.2
       Component:  Compiler (Type    |                 Version:  7.10.1-rc1
  checker)                           |                Keywords:
      Resolution:                    |            Architecture:
Operating System:  Unknown/Multiple  |  Unknown/Multiple
 Type of failure:  GHC rejects       |               Test Case:
  valid program                      |                Blocking:
      Blocked By:                    |  Differential Revisions:
 Related Tickets:  #10226            |
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Comment (by jeltsch):

 I have an issue with GHC 7.10.1, which is probably rooted in the same bug.
 The following code is accepted by GHC 7.8.3, but not 7.10.1:

 {{{
 {-# LANGUAGE TypeFamilies #-}

 type family F a :: *

 type family G b :: *

 x :: G (F a) ~ a => F a
 x = undefined
 }}}

 GHC 7.10.1 gives the following error message:

 {{{
 Could not deduce (F a0 ~ F a)
 from the context (G (F a) ~ a)
   bound by the type signature for x :: (G (F a) ~ a) => F a
   at Test.hs:7:6-23
 NB: ‘F’ is a type function, and may not be injective
 The type variable ‘a0’ is ambiguous
 In the ambiguity check for the type signature for ‘x’:
   x :: forall a. (G (F a) ~ a) => F a
 To defer the ambiguity check to use sites, enable AllowAmbiguousTypes
 In the type signature for ‘x’: x :: G (F a) ~ a => F a
 }}}

 Is this code accepted by GHC HEAD?

 I came across this problem when trying to recompile the `incremental-
 computing` package with GHC 7.10.1. Since this issue breaks `incremental-
 computing` in a nontrivial way, I would be ''extremely'' happy if the fix
 of this bug would make it into GHC 7.10.2.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10009#comment:29>
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