[GHC] #15079: GHC HEAD regression: cannot instantiate higher-rank kind

[GHC]

#15079: GHC HEAD regression: cannot instantiate higher-rank kind
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        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  high              |            Milestone:  8.6.1
       Component:  Compiler (Type    |              Version:  8.5
  checker)                           |
      Resolution:                    |             Keywords:  TypeInType
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):

 Now that I have a better understanding of why this is failing, here's a
 simpler program which demonstrates the issue (this typechecks on GHC
 8.4.2, but not HEAD):

 {{{#!hs
 {-# LANGUAGE RankNTypes #-}
 {-# LANGUAGE TypeInType #-}
 module Bug where

 import Data.Kind

 newtype Foo (f :: forall (a :: Type). a -> Type) = MkFoo (f Int)
 data InferredProxy a = MkInferredProxy

 foo :: Foo InferredProxy
 foo = MkFoo MkInferredProxy
 }}}
 {{{
 $ ~/Software/ghc/inplace/bin/ghc-stage2 Bug.hs
 [1 of 1] Compiling Bug              ( Bug.hs, Bug.o )

 Bug.hs:11:13: error:
     • Kind mismatch: cannot unify (f0 :: forall a. a -> *) with:
         InferredProxy :: forall k. k -> *
       Their kinds differ.
       Expected type: f0 * Int
         Actual type: InferredProxy Int
     • In the first argument of ‘MkFoo’, namely ‘MkInferredProxy’
       In the expression: MkFoo MkInferredProxy
       In an equation for ‘foo’: foo = MkFoo MkInferredProxy
    |
 11 | foo = MkFoo MkInferredProxy
    |             ^^^^^^^^^^^^^^^
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15079#comment:5>
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