[GHC] #14394: Inferred type for pattern synonym has redundant equality constraint

[GHC]

#14394: Inferred type for pattern synonym has redundant equality constraint
-------------------------------------+-------------------------------------
           Reporter:  RyanGlScott    |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.2.1
  (Type checker)                     |
           Keywords:                 |  Operating System:  Unknown/Multiple
  PatternSynonyms, TypeInType        |
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 Load this file into GHCi:

 {{{#!hs
 {-# LANGUAGE GADTs #-}
 {-# LANGUAGE PatternSynonyms #-}
 module Bug where

 import Data.Type.Equality

 pattern Foo = HRefl
 }}}
 {{{
 $ /opt/ghc/8.2.1/bin/ghci Bug.hs
 GHCi, version 8.2.1: http://www.haskell.org/ghc/  :? for help
 Loaded GHCi configuration from /home/rgscott/.ghci
 [1 of 1] Compiling Bug              ( Bug.hs, interpreted )
 Ok, 1 module loaded.
 λ> :i Foo
 pattern Foo :: () => (* ~ *, b ~ a) => a :~~: b
 }}}

 Notice that the type signature for `Foo` has an entirely redundant `* ~ *`
 constraint. The same does not happen if `TypeInType` is enabled.

 {{{#!hs
 {-# LANGUAGE GADTs #-}
 {-# LANGUAGE PatternSynonyms #-}
 {-# LANGUAGE TypeInType #-}
 module Works where

 import Data.Type.Equality

 pattern Foo = HRefl
 }}}
 {{{
 λ> :i Foo
 pattern Foo :: forall k2 k1 (a :: k1) (b :: k2). () => (k2 ~ k1,
                                                         (b :: k2) ~~ (a ::
 k1)) => a :~~: b
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14394>
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