[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 Haskell  |              Version:  8.6.1
      Resolution:                    |             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:                    |
-------------------------------------+-------------------------------------
Description changed by int-e:

Old description:

> 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.

New description:

 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
 23956b2ada690c78a134fe6d149940c777c7efcc), 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#comment:1>
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