[GHC] #11225: Unable to provide type signature for pattern synonym

[GHC]

#11225: Unable to provide type signature for pattern synonym
-------------------------------------+-------------------------------------
           Reporter:  mpickering     |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  7.10.3
           Keywords:                 |  Operating System:  Unknown/Multiple
  PatternSynonyms                    |
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:  patsyn/T11224  |        Blocked By:
           Blocking:                 |   Related Tickets:  #11224
Differential Rev(s):                 |         Wiki Page:
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 {{{
 {-# language
    PatternSynonyms
  , ViewPatterns
  #-}

 import Text.Read

 pattern PRead a <- (readMaybe -> Just a)
 }}}

 GHC infers the type correctly for `PRead` but it is impossible provide a
 signature. See #11224 for more discussion about why.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11225>
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