[GHC] #12655: Bizarre parser problem: "Illegal bang-pattern" (something to do with CPP?)

3 Oct 2016

      #12655: Bizarre parser problem: "Illegal bang-pattern" (something to do with CPP?)
-------------------------------------+-------------------------------------
           Reporter:  edsko          |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  7.4.2
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 This is a bug in an old ghc (7.4.2), documenting it here mostly in case
 anybody else comes across it. If you try to compile
 http://hackage.haskell.org/package/bytestring-builder-0.10.8.1.0
 /bytestring-builder-0.10.8.1.0.tar.gz , ghc fails with

 {{{
 src/Data/ByteString/Builder/Prim.hs:553:13:
     Illegal bang-pattern (use -XBangPatterns):
     ! br'
 }}}

 However, that file contains a directive

 {{{#!hs
 {-# LANGUAGE CPP, BangPatterns, ScopedTypeVariables #-}
 }}}

 at the very top of the file, so we should not be getting this error
 message. Now here's the bizarre thing: if we compile with `cabal
 --builddir=foo`, compilation happens as normal! I ran ghc with `--keep-
 tmp-files`, and compared the output of CPP for both variations, and
 literally the ''only'' difference between the two that lines

 {{{
 # 156 "./dist/build/autogen/cabal_macros.h"
 }}}

 in one are

 {{{
 # 156 "./foo/build/autogen/cabal_macros.h"
 }}}

 in the other.

 It gets weirder. I replaced the output of
 `src/Data/ByteString/Builder/Prim.hs` with the output of `cpp`, and
 removed the `CPP` language directive. If I start with the version that has
 `dist` (i.e., the one that results in a compilation error), ''even adding
 or removing a single blank line in the header of the file made the parser
 error go away'' (where the "header" is the bit with lines such as

 {{{
 # 134 "./dist/build/autogen/cabal_macros.h"
 }}}

 No idea what's going on here; probably not worth investigating further.

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12655
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