[GHC] #15815: problem with splicing type into constraint

[GHC]

#15815: problem with splicing type into constraint
-------------------------------------+-------------------------------------
           Reporter:  int-e          |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Template       |           Version:  8.6.1
  Haskell                            |
           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:
-------------------------------------+-------------------------------------
 Consider the following two-module example. (as gist:
 https://gist.github.com/int-e/a666991423c10150bd99dd0e874d6150)
 {{{#!hs
 {-# LANGUAGE TemplateHaskell #-}
 module A where

 mkFoo tyQ = [d|
     foo :: a ~ $(tyQ) => a
     foo = undefined
 |]
 }}}

 {{{#!hs
 {-# LANGUAGE TemplateHaskell, GADTs #-}
 module B where

 import A

 mkFoo [t| Int -> Int |]
 }}}

 This loads fine in ghc-8.4.2, but with ghc-8.6.1 and current head (commit
 578012be13eb1548050d51c0a23bd1a98423f03e), the splice goes wrong:
 {{{
 $ ghci B.hs
 GHCi, version 8.6.1: http://www.haskell.org/ghc/  :? for help
 [1 of 2] Compiling A                ( A.hs, interpreted )
 [2 of 2] Compiling B                ( B.hs, interpreted )

 B.hs:7:1: error:
     • Expected a type, but ‘a_a4uD ~ Int’ has kind ‘Constraint’
     • In the type signature: foo :: a_a4uD ~ Int -> Int => a_a4uD
   |
 7 | mkFoo [t| Int -> Int |]
   | ^^^^^^^^^^^^^^^^^^^^^^^
 Failed, one module loaded.
 *A>
 }}}

 As a workaround one can define a type alias for the `Int -> Int` type.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15815>
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