Re: [GHC] #14883: QuantifiedConstraints don't kick in when used in TypeApplications

2 Mar 2018

      #14883: QuantifiedConstraints don't kick in when used in TypeApplications
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        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  8.5
  checker)                           |             Keywords:
      Resolution:                    |  QuantifiedConstraints, wipT2893
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC rejects       |  Unknown/Multiple
  valid program                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by RyanGlScott):

 Another example in the same vein:

 {{{#!hs
 {-# LANGUAGE ConstraintKinds #-}
 {-# LANGUAGE ImpredicativeTypes #-}
 {-# LANGUAGE InstanceSigs #-}
 {-# LANGUAGE KindSignatures #-}
 {-# LANGUAGE QuantifiedConstraints #-}
 {-# LANGUAGE RankNTypes #-}
 {-# LANGUAGE ScopedTypeVariables #-}
 {-# LANGUAGE TypeApplications #-}
 module Bug where

 import Data.Coerce
 import Data.Kind

 type Phantom1 p = (forall a b. Coercible (p a) (p b) :: Constraint)

 class Foo a where
   bar :: Phantom1 proxy => proxy a -> Int

 instance Foo Int where
   bar _ = 42

 -- Typecheck
 newtype Age1 = MkAge1 Int
 instance Foo Age1 where
   bar :: forall proxy. Phantom1 proxy => proxy Age1 -> Int
   bar = coerce @(proxy Int  -> Int)
                @(proxy Age1 -> Int)
                bar

 -- Doesn't typecheck
 newtype Age2 = MkAge2 Int
 instance Foo Age2 where
   bar = coerce @(forall proxy. Phantom1 proxy => proxy Int  -> Int)
                @(forall proxy. Phantom1 proxy => proxy Age2 -> Int)
                bar
 }}}

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14883#comment:1
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