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

[GHC]

#15815: problem with splicing type into constraint
-------------------------------------+-------------------------------------
        Reporter:  int-e             |                Owner:  RyanGlScott
            Type:  bug               |               Status:  new
        Priority:  highest           |            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:                    |
-------------------------------------+-------------------------------------

Comment (by RyanGlScott):

 Simpler example that only requires one module:

 {{{#!hs
 {-# LANGUAGE GADTs #-}
 {-# LANGUAGE TemplateHaskell #-}
 module Bug where

 $([d| foo :: a ~ (Int -> Int) => a
       foo = undefined |])
 }}}
 {{{
 $ /opt/ghc/8.4.4/bin/ghc Bug.hs
 [1 of 1] Compiling Bug              ( Bug.hs, Bug.o )

 $ /opt/ghc/8.6.1/bin/ghc Bug.hs
 [1 of 1] Compiling Bug              ( Bug.hs, Bug.o )

 Bug.hs:5:3: error:
     • Expected a type, but ‘a_a44c ~ Int’ has kind ‘Constraint’
     • In the type signature: foo :: a_a44c ~ Int -> Int => a_a44c
   |
 5 | $([d| foo :: a ~ (Int -> Int) => a
   |   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^...
 }}}

 Or, using fewer quotes:

 {{{#!hs
 {-# LANGUAGE GADTs #-}
 {-# LANGUAGE TemplateHaskell #-}
 module Bug where

 import Language.Haskell.TH

 $(do foo <- newName "foo"
      a   <- newName "a"
      pure [ SigD foo (ForallT [] [AppT (AppT EqualityT (VarT a)) (AppT
 (AppT ArrowT (ConT ''Int)) (ConT ''Int))] (VarT a))
           , ValD (VarP foo) (NormalB (VarE 'undefined)) []
           ])
 }}}
 {{{
 $ /opt/ghc/8.4.4/bin/ghc Bug.hs
 [1 of 1] Compiling Bug              ( Bug.hs, Bug.o )

 $ /opt/ghc/8.6.1/bin/ghc Bug.hs
 [1 of 1] Compiling Bug              ( Bug.hs, Bug.o )

 Bug.hs:7:3: error:
     • Expected a type, but ‘a_a452 ~ Int’ has kind ‘Constraint’
     • In the type signature: foo :: a_a452 ~ Int -> Int => a_a452
   |
 7 | $(do foo <- newName "foo"
   |   ^^^^^^^^^^^^^^^^^^^^^^^...
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15815#comment:3>
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