Bizarre parser problem: "Illegal bang-pattern" (something to do with CPP?)

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

{-# 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.

Trac metadata

Trac field	Value
Version	7.4.2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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