[GHC] #15144: Type inference regression between GHC 8.0.2 and 8.2.2

[GHC]

#15144: Type inference regression between GHC 8.0.2 and 8.2.2
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        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.6.1
       Component:  Compiler (Type    |              Version:  8.2.2
  checker)                           |
      Resolution:                    |             Keywords:  TypeFamilies
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Changes (by RyanGlScott):

 * keywords:   => TypeFamilies


Comment:

 Here's a slightly smaller test case:

 {{{#!hs
 {-# LANGUAGE FlexibleContexts #-}
 {-# LANGUAGE TypeFamilies #-}
 module Bug where

 import Data.Coerce
 import Data.Proxy

 type family F x

 f :: Coercible (F a) b => Proxy a -> F a -> b
 f _ = coerce

 g p x = f p x
 }}}
 {{{
 $ /opt/ghc/8.0.2/bin/ghci Bug.hs
 GHCi, version 8.0.2: http://www.haskell.org/ghc/  :? for help
 Loaded GHCi configuration from /home/rgscott/.ghci
 [1 of 1] Compiling Bug              ( Bug.hs, interpreted )
 Ok, modules loaded: Bug.
 λ> :t g
 g :: Coercible b (F a) => Proxy a -> F a -> b
 }}}
 {{{
 $ /opt/ghc/8.4.2/bin/ghci Bug.hs
 GHCi, version 8.4.2: http://www.haskell.org/ghc/  :? for help
 Loaded GHCi configuration from /home/rgscott/.ghci
 [1 of 1] Compiling Bug              ( Bug.hs, interpreted )
 Ok, one module loaded.
 λ> :t g
 g :: Proxy a -> F a -> F a
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15144#comment:1>
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