[GHC] #13782: Bullish use of Template Haskell's newName causes GHC internal error

2 Jun 2017

      #13782: Bullish use of Template Haskell's newName causes GHC internal error
-------------------------------------+-------------------------------------
           Reporter:  RyanGlScott    |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Template       |           Version:  8.0.1
  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:
-------------------------------------+-------------------------------------
 int-index originally spotted this bug at
 https://github.com/goldfirere/singletons/issues/150#issuecomment-305909199.
 To reproduce, compile this file with GHC 8.0.1, 8.0.2, 8.2.1, or HEAD:

 {{{#!hs
 {-# LANGUAGE DataKinds #-}
 {-# LANGUAGE ExistentialQuantification #-}
 {-# LANGUAGE PolyKinds #-}
 {-# LANGUAGE TemplateHaskell #-}
 module Bug where

 import Language.Haskell.TH

 $(do TyConI (DataD _ _ [KindedTV a1 _] _ _ _) <- reify ''Maybe
      [f,a2] <- mapM newName ["f","a"]
      return [ SigD f (ForallT [KindedTV a2 (AppT (ConT ''Maybe) (VarT
 a1))]
                    [] (ConT ''Int))
             , ValD (VarP f) (NormalB (LitE (IntegerL 42))) []
             ])
 }}}

 {{{
 GHCi, version 8.2.0.20170522: http://www.haskell.org/ghc/  :? for help
 Loaded GHCi configuration from /home/rgscott/.ghci
 [1 of 1] Compiling Bug              ( Bug.hs, interpreted )

 Bug.hs:9:3: error:
     • GHC internal error: ‘a_11’ is not in scope during type checking, but
 it passed the renamer
       tcl_env of environment: []
     • In the first argument of ‘Maybe’, namely ‘a_11’
       In the kind ‘Maybe a_11’
       In the type signature: f :: forall (a_a4Qz :: Maybe a_11). Int
   |
 9 | $(do TyConI (DataD _ _ [KindedTV a1 _] _ _ _) <- reify ''Maybe
   |   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^...
 }}}

 The root cause of the issue seems to be that the name `a` (which we picked
 for `newName`) happens to clash with the type variable we reified from
 `Maybe` (since `data Maybe a = ...`). If we pick a different name:

 {{{#!hs
      [f,a2] <- mapM newName ["f","albatross"]
 }}}

 Then it will compile.

 This is a regression from GHC 7.10.3, as it compiles in that version (with
 a slight change to accommodate the API differences in `DataD` between
 7.10.3 and 8.0):

 {{{#!hs
 {-# LANGUAGE DataKinds #-}
 {-# LANGUAGE ExistentialQuantification #-}
 {-# LANGUAGE PolyKinds #-}
 {-# LANGUAGE TemplateHaskell #-}
 module Works where

 import Language.Haskell.TH

 $(do TyConI (DataD _ _ [KindedTV a1 _] _ _) <- reify ''Maybe
      [f,a2] <- mapM newName ["f","a"]
      return [ SigD f (ForallT [KindedTV a2 (AppT (ConT ''Maybe) (VarT
 a1))]
                    [] (ConT ''Int))
             , ValD (VarP f) (NormalB (LitE (IntegerL 42))) []
             ])
 }}}

 {{{
 $ /opt/ghc/7.10.3/bin/ghci Works.hs -ddump-splices
 GHCi, version 7.10.3: http://www.haskell.org/ghc/  :? for help
 [1 of 1] Compiling Works            ( Works.hs, interpreted )
 Works.hs:(9,3)-(14,13): Splicing declarations
     do { TyConI (DataD _ _ [KindedTV a1_a3jt _] _ _) <- reify ''Maybe;
          [f_a3nh, a2_a3ni] <- mapM newName ["f", "a"];
          return
            [SigD
               f_a3nh
               (ForallT
                  [KindedTV a2_a3ni (AppT (ConT ''Maybe) (VarT a1_a3jt))]
                  []
                  (ConT ''Int)),
             ValD (VarP f_a3nh) (NormalB (LitE (IntegerL 42))) []] }
   ======>
     f_a4vc :: forall (a_a4vd :: Maybe a_a4v3). Int
     f_a4vc = 42
 Ok, modules loaded: Works.
 }}}

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13782
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