[GHC] #12819: Pattern synonym signatures are not validity-checked

[GHC]

#12819: Pattern synonym signatures are not validity-checked
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           Reporter:  goldfire       |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.1
  (Type checker)                     |
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  GHC accepts
  Unknown/Multiple                   |  invalid program
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 Ponder this garbage:

 {{{#!hs
 {-# LANGUAGE PatternSynonyms, ViewPatterns,
              TypeFamilies, KindSignatures #-}

 module Bug where

 type family F a  -- F :: * -> *
 data T :: (* -> *) -> *

 pattern Q :: T F -> String
 pattern Q x <- (undefined -> x)
 }}}

 This is accepted by GHC 8.0.1 and HEAD. But it has an unsaturated type
 family! Urk!

 The problem is that `TcSigs.tcPatSynSig` never does validity checking.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12819>
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