bug#12142: automake tries to compile a program when 'foo' and 'foo.cxx' exist (though the former is header)

[Michał Górny]

autoconf: 2.69
automake: 1.12.2
libtool: 2.4.2

Hello,

I was writing a C++ library and I think I hit a pretty ugly bug within
automake. My library was structured like the following:

- src/foo (the header file),
- src/foo.cxx (the code).

These files assemble a library, let's call it libfoo.la.

The problem is that every time I modify src/foo.cxx and call 'make',
automake first (correctly) compiles the library, and then mistakenly
tries to compile and link it into a program src/foo, either effectively
overwriting or removing my header file, or failing early due to
missing dependency file.

What's interesting, 'make' calls not following src/foo.cxx changes work
correctly; just the one after modifying src/foo.cxx fails.

Here are the minimal configure.ac & Makefile.am files with which I can
reproduce the issue:


configure.ac:

AC_PREREQ([2.60])
AC_INIT([foo], [0])
AM_INIT_AUTOMAKE([1.6 foreign])

AC_PROG_CXX
LT_INIT

AC_CONFIG_HEADERS([config.h])
AC_CONFIG_FILES([Makefile])
AC_OUTPUT


Makefile.am:

include_HEADERS = src/foo
lib_LTLIBRARIES = libfoo.la

libfoo_la_SOURCES = src/foo.cxx


I've also prepared a git repository with the simple test case:

git clone https://bitbucket.org/mgorny/automake-cxx-bug.git
cd automake-cxx-bug
autoreconf -vi
./configure --disable-static
make # compiles fine
touch src/foo.cxx
make # fails


Related Gentoo bug report:
https://bugs.gentoo.org/show_bug.cgi?id=429972

-- 
Best regards,
Michał Górny

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