[GHC] #11053: Add a warning for pattern synonyms without signatures

[GHC]

#11053: Add a warning for pattern synonyms without signatures
-------------------------------------+-------------------------------------
        Reporter:  mpickering        |                Owner:  mpickering
            Type:  feature request   |               Status:  closed
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  7.10.2
      Resolution:  fixed             |             Keywords:  newcomer
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
                                     |  patsyns/T11053
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by goldfire):

 Note that including this in `-Wall` makes it harder for clients to
 maintain a warning-free library over 3 releases, given that pattern
 synonyms were available one cycle before their signatures. I'm still not
 personally entirely convinced about that policy, but it does seem to have
 traction and we should keep it in mind as we evolve the language.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11053#comment:4>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler