[GHC] #14921: Type inference breaks on constraint-kinded parameter used by a Rank-2 callback

[GHC]

#14921: Type inference breaks on constraint-kinded parameter used by a Rank-2
callback
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        Reporter:  glittershark      |                Owner:  (none)
            Type:  bug               |               Status:  closed
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.2.2
      Resolution:  invalid           |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC rejects       |  Unknown/Multiple
  valid program                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #10651, #15649    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by infinity0):

 As mentioned on the other tickets `Proxy` is an alternative workaround to
 `TypeApplications` + `AllowAmbiguousTypes`, the latter being quite a
 strong relaxation that one often doesn't want to enable in library code.

 {{{#!haskell
 {-# LANGUAGE RankNTypes #-}
 {-# LANGUAGE ConstraintKinds #-}
 {-# LANGUAGE ScopedTypeVariables #-}

 module Scratch where

 import Data.Proxy
 import Prelude

 data Foo
   = FooA Int
   | FooB String

 useFoo :: forall cls a . (cls Int, cls String) => Proxy cls -> (forall k .
 cls k => k -> a) -> Foo -> a
 useFoo _ f (FooA a) = f a
 useFoo _ f (FooB b) = f b

 x = useFoo (Proxy :: Proxy Show) show (FooA 1)
 }}}

 compiles without errors.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14921#comment:3>
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