[GHC] #12513: Template Haskell boxes singleton unboxed tuples when splicing them

[GHC]

#12513: Template Haskell boxes singleton unboxed tuples when splicing them
-------------------------------------+-------------------------------------
           Reporter:  RyanGlScott    |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Template       |           Version:  8.0.1
  Haskell                            |
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  GHC accepts
  Unknown/Multiple                   |  invalid program
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:  #5332
Differential Rev(s):                 |         Wiki Page:
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 As noticed [https://phabricator.haskell.org/D2448#71603 here], this
 program somehow compiles:

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

 import Language.Haskell.TH.Lib
 import Language.Haskell.TH.Ppr
 import Language.Haskell.TH.Syntax

 f :: $([t| (# Int #) |]) -> Int
 f x = x

 g :: $(unboxedTupleT 1 `appT` conT ''Int) -> Int
 g x = x
 }}}

 Despite the fact that `(# Int #)` and `Int` are most definitely //not//
 the same type! If you compile with `-ddump-splices`, you'll see what's
 going on:

 {{{
 $ /opt/ghc/head/bin/ghc Bug.hs -ddump-splices
 [1 of 1] Compiling Bug              ( Bug.hs, Bug.o )
 Bug.hs:12:8-40: Splicing type
     unboxedTupleT 1 `appT` conT ''Int ======> Int
 Bug.hs:9:8-23: Splicing type [t| (# Int #) |] ======> In
 }}}

 It appears that the splicing machinery is turning `(# Int #)` into `Int`
 behind the scenes. Luckily, this should be easy to fix. Patch coming soon.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12513>
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