[GHC] #12975: Suggested type signature for a pattern synonym causes program to fail to type check

[GHC]

#12975: Suggested type signature for a pattern synonym causes program to fail to
type check
-------------------------------------+-------------------------------------
        Reporter:  ocharles          |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  8.0.1
  checker)                           |             Keywords:
      Resolution:                    |  patternsynonyms
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by ocharles):

 I actually believe the correct type of this pattern is

 {{{#!hs
 pattern Cast :: () => Typeable a => a -> T
 pattern Cast a <- MkT (cast -> Just a)
 }}}

 But this is also rejected:

 {{{
     • Could not deduce (Typeable a0) arising from a use of ‘cast’
       from the context: Typeable a
         bound by a pattern with constructor:
                    MkT :: forall a. Typeable a => a -> T,
                  in a pattern synonym declaration
         at ../foo.hs:9:19-38
       The type variable ‘a0’ is ambiguous
     • In the pattern: cast -> Just a
       In the pattern: MkT (cast -> Just a)
       In the declaration for pattern synonym ‘Cast’
 }}}

 I may be wrong with that type though.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12975#comment:1>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler