[GHC] #12069: Possible type inference regression: "Expected a type, but 'a' has kind 'k0'"

16 May 2016

      #12069: Possible type inference regression: "Expected a type, but 'a' has kind
'k0'"
-------------------------------------+-------------------------------------
           Reporter:  akio           |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.1
  (Type checker)                     |
           Keywords:                 |  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:
-------------------------------------+-------------------------------------
 The following program causes GHC HEAD to give an error:

 {{{#!hs
 {-# LANGUAGE ScopedTypeVariables #-}
 module Foo where

 foo (_ :: p a) = [] :: [a]
 }}}

 {{{
 [1 of 1] Compiling Foo              ( kind.hs, kind.o )

 kind.hs:4:25: error:
     • Expected a type, but ‘a’ has kind ‘k0’
     • In an expression type signature: [a]
       In the expression: [] :: [a]
       In an equation for ‘foo’: foo (_ :: p a) = [] :: [a]
 }}}

 GHC 7.10.1 compiles it fine, and infers the type `foo :: p a -> [a]`.

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Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12069
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