[GHC] #14288: ScopedTypeVariables with nested foralls broken since 8.0.2

[GHC]

#14288: ScopedTypeVariables with nested foralls broken since 8.0.2
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        Reporter:  MikolajKonarski   |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  8.2.1
  checker)                           |
      Resolution:                    |             Keywords:
Operating System:  Linux             |         Architecture:  x86_64
 Type of failure:  GHC rejects       |  (amd64)
  valid program                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by RyanGlScott):

 Actually, the issue I mentioned about pattern synonyms applies //today//.
 That it, this typechecks:

 {{{#!hs
 {-# LANGUAGE GADTs #-}
 {-# LANGUAGE PatternSynonyms #-}
 {-# LANGUAGE ScopedTypeVariables #-}
 {-# LANGUAGE TypeApplications #-}

 data T a where
   MkT :: forall a b. (Show b) => a -> b -> T a

 pattern ExNumPat :: forall a. (Num a, Eq a) => forall b. (Show b) => b ->
 T a
 pattern ExNumPat x <- MkT 42 x where
   ExNumPat x = MkT @a 42 x
 }}}

 But this (with `@b`) doesn't!

 {{{#!hs
 {-# LANGUAGE GADTs #-}
 {-# LANGUAGE PatternSynonyms #-}
 {-# LANGUAGE ScopedTypeVariables #-}
 {-# LANGUAGE TypeApplications #-}

 data T a where
   MkT :: forall a b. (Show b) => a -> b -> T a

 pattern ExNumPat :: forall a. (Num a, Eq a) => forall b. (Show b) => b ->
 T a
 pattern ExNumPat x <- MkT 42 x where
   ExNumPat x = MkT @a @b 42 x
 }}}

 Moreover, there's no workaround for this, since one can't combine the two
 `forall`s in the pattern signature for the reasons discussed in comment:6.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14288#comment:11>
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