[GHC] #12853: Unpromoted tuples in TH in correctly accepted by tthe type checker

[GHC]

#12853: Unpromoted tuples in TH in correctly accepted by tthe type checker
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        Reporter:  erikd             |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.2.1
       Component:  Compiler (Type    |              Version:  8.1
  checker)                           |
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  Other             |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Description changed by erikd:

@@ -14,1 +14,0 @@
- class C (a :: Bool)
@@ -16,4 +15,0 @@
- class E (a :: [Bool])
-
- instance C True
- instance C 'False
@@ -23,3 +18,0 @@
-
- instance E '[True, False]
- instance E '[False, True]

New description:

 This issue was discovered when writing tests for #11629.

 Test case:
 {{{#!hs
 {-# LANGUAGE DataKinds #-}
 {-# LANGUAGE TemplateHaskell #-}
 {-# LANGUAGE KindSignatures #-}
 {-# LANGUAGE FlexibleInstances #-}
 module T11629 where

 import Control.Monad
 import Language.Haskell.TH

 class D (a :: (Bool, Bool))

 instance D '(True, False)
 instance D '(False, True)

 do
   let getType (InstanceD _ _ ty _) = ty
       getType _                    = error "getType: only defined for
 InstanceD"

       failMsg a ty1 ty2 = fail $ "example " ++ a
         ++ ": ty1 /= ty2, where\n ty1 = "
         ++ show ty1 ++ "\n ty2 = " ++ show ty2

       withoutSig (ForallT tvs cxt ty) = ForallT tvs cxt (withoutSig ty)
       withoutSig (AppT ty1 ty2)       = AppT (withoutSig ty1) (withoutSig
 ty2)
       withoutSig (SigT ty ki)         = withoutSig ty
       withoutSig ty                   = ty

   -- test #2: type quotations and reified types should agree wrt
   -- promoted tuples.
   ty1 <- [t| D '(True, False) |]
   ty2 <- [t| D (False, True)  |]
   ClassI _ insts <- reify ''D
   let [ty1', ty2'] = map (withoutSig . getType) insts

   when (ty1 /= ty1') $ failMsg "C" ty1 ty1'
   when (ty2 /= ty2') $ failMsg "D" ty2 ty2'

   return []
 }}}

 According to @RyanlGlScott in https://phabricator.haskell.org/D2188#79228
 the code `D (True, False)` in regular Haskell code would be rejected as a
 Kind error, whereas when its in Oxford brackets like `[t| D (True, False)
 |]`, it manages to sneak through the type checker.

--

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12853#comment:1>
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