[GHC] #14160: Type inference breaking change in GHC 8.0.2

[GHC]

#14160: Type inference breaking change in GHC 8.0.2
-------------------------------------+-------------------------------------
        Reporter:  Iceland_jack      |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.2.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------
Description changed by Iceland_jack:

Old description:

> A regression reported by
> [https://www.reddit.com/r/haskell/comments/6w7grz/type_inference_breaking_change_in_ghc_802/
> Milewski],
>
> {{{#!hs
> {-# LANGUAGE RankNTypes #-}
> module Test where
>
> import Data.Profunctor
>
> proj :: Profunctor p => forall c. (forall a. p a a) -> p c c
> proj e = e
>
> f1 :: Profunctor p => (a -> b) -> (forall c. p c c) -> p a b
> f1 f e = dimap f id (proj e)
> }}}
>
> The regression is that these used to work, but do not currently
>
> {{{#!hs
> -- • Couldn't match type ‘p c0 c0’ with ‘forall a1. p a1 a1’
> --   Expected type: p c0 c0 -> p a a
> --     Actual type: (forall a1. p a1 a1) -> p a a
> -- • In the second argument of ‘(.)’, namely ‘proj’
> --   In the expression: dimap id f . proj
> --   In an equation for ‘f2’: f2 f = dimap id f . proj
> -- • Relevant bindings include
> --     f2 :: (a -> b) -> (forall c. p c c) -> p a b
> --       (bound at 24:1)
> f2 :: Profunctor p => (a -> b) -> (forall c. p c c) -> p a b
> f2 f = dimap id f . proj
>
> -- • Cannot instantiate unification variable ‘a0’
> --   with a type involving foralls: (forall c. p c c) -> p a b
> --     GHC doesn't yet support impredicative polymorphism
> -- • In the expression: undefined
> --   In an equation for ‘f3’: f3 f = undefined
> -- • Relevant bindings include
> --     f :: a -> b
> --       (bound at 39:4)
> --     f3 :: (a -> b) -> (forall c. p c c) -> p a b
> --       (bound at 39:1)
>
> f3 :: Profunctor p => (a -> b) -> (forall c. p c c) -> p a b
> f3 f = undefined -- dimap id f . proj
> }}}

New description:

 A regression reported by
 [https://www.reddit.com/r/haskell/comments/6w7grz/type_inference_breaking_change_in_ghc_802/
 Milewski],

 {{{#!hs
 {-# LANGUAGE RankNTypes #-}
 module Test where

 import Data.Profunctor

 proj :: Profunctor p => forall c. (forall a. p a a) -> p c c
 proj e = e

 f1 :: Profunctor p => (a -> b) -> (forall c. p c c) -> p a b
 f1 f e = dimap f id (proj e)
 }}}

 Where these definitions no longer type check

 {{{#!hs
 -- • Couldn't match type ‘p c0 c0’ with ‘forall a1. p a1 a1’
 --   Expected type: p c0 c0 -> p a a
 --     Actual type: (forall a1. p a1 a1) -> p a a
 -- • In the second argument of ‘(.)’, namely ‘proj’
 --   In the expression: dimap id f . proj
 --   In an equation for ‘f2’: f2 f = dimap id f . proj
 -- • Relevant bindings include
 --     f2 :: (a -> b) -> (forall c. p c c) -> p a b
 --       (bound at 24:1)
 f2 :: Profunctor p => (a -> b) -> (forall c. p c c) -> p a b
 f2 f = dimap id f . proj

 -- • Cannot instantiate unification variable ‘a0’
 --   with a type involving foralls: (forall c. p c c) -> p a b
 --     GHC doesn't yet support impredicative polymorphism
 -- • In the expression: undefined
 --   In an equation for ‘f3’: f3 f = undefined
 -- • Relevant bindings include
 --     f :: a -> b
 --       (bound at 39:4)
 --     f3 :: (a -> b) -> (forall c. p c c) -> p a b
 --       (bound at 39:1)

 f3 :: Profunctor p => (a -> b) -> (forall c. p c c) -> p a b
 f3 f = undefined -- dimap id f . proj
 }}}

--

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14160#comment:1>
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