[CMake] enable_language(.... OPTIONAL) signature does not work correctly

[Alan W. Irwin]

I have been testing the OPTIONAL signature for enable_language using the
following simple CMakeLists.txt file:

*******
cmake_minimum_required(VERSION 2.6.4)
project(test NONE)
enable_language(C OPTIONAL)
message(STATUS "CMAKE_C_COMPILER_WORKS = ${CMAKE_C_COMPILER_WORKS}")
*******

According to the documentation the OPTIONAL signature is to help CMake-based
build systems deal smoothly with bad/broken compilers by allowing them to
take their own actions to deal with the compiler issue based on the value of
CMAKE_<languageName>_COMPILER_WORKS.  However, the OPTIONAL signature does
not work and bad/broken compilers error out before the above message command
can be reached.  The same issue also occurs for C++ and Fortran.

To illustrate the issue as simply as possible, I emulate a missing/broken
compiler as follows:

software at raven> CC='gcc whatever' cmake ..
-- The C compiler identification is unknown
-- Check for working C compiler: /usr/bin/gcc
-- Check for working C compiler: /usr/bin/gcc -- broken
CMake Error at /home/software/cmake/install-2.6.4/share/cmake-2.6/Modules/CMakeTestCCompiler.cmake:32 (MESSAGE):
   The C compiler "/usr/bin/gcc" is not able to compile a simple test program.

   It fails with the following output:

    Change Dir: /tmp/build_dir/CMakeFiles/CMakeTmp



   Run Build Command:/usr/bin/make "cmTryCompileExec/fast"

   /usr/bin/make -f CMakeFiles/cmTryCompileExec.dir/build.make
   CMakeFiles/cmTryCompileExec.dir/build

   make[1]: Entering directory `/tmp/build_dir/CMakeFiles/CMakeTmp'

   /home/software/cmake/install-2.6.4/bin/cmake -E cmake_progress_report
   /tmp/build_dir/CMakeFiles/CMakeTmp/CMakeFiles 1

   Building C object CMakeFiles/cmTryCompileExec.dir/testCCompiler.c.o

   /usr/bin/gcc whatever -o CMakeFiles/cmTryCompileExec.dir/testCCompiler.c.o
   -c /tmp/build_dir/CMakeFiles/CMakeTmp/testCCompiler.c

   gcc: whatever: No such file or directory

   make[1]: Leaving directory `/tmp/build_dir/CMakeFiles/CMakeTmp'

   make[1]: *** [CMakeFiles/cmTryCompileExec.dir/testCCompiler.c.o] Error 1

   make: *** [cmTryCompileExec/fast] Error 2





   CMake will not be able to correctly generate this project.
Call Stack (most recent call first):
   CMakeLists.txt:3 (enable_language)


-- Configuring incomplete, errors occurred!

The "whatever" inserts an extra unknown file into the compilation process
which makes try_compile not work, and which therefore tests the OPTIONAL
signature.  However, instead of returning properly to the original
CMakeLists.txt file to allow the build system to do something about
CMAKE_C_COMPILER_WORKS being false it errors out as above.

The source of the issue is line 32 of CMakeTestCCompiler.cmake which
uses message(FATAL_ERROR ...)

If you change that to message(STATUS ...) , then that solves the issue and
control is returned back to the originating CMakeLists.txt file with this
(correct) result

[...]
CMake will not be able to correctly generate this project.
-- CMAKE_C_COMPILER_WORKS = FALSE
-- Configuring done
-- Generating done
-- Build files have been written to: /tmp/build_dir

However, that is not a good general solution because the same fix means that
if you drop OPTIONAL, i.e., use "enable_language(C)" control is incorrectly
returned to the original CMakeLists.txt and the project is incorrectly
configured and generated without a CMake error.

I had a look at cmEnableLanguageCommand.cxx to see what it did with
OPTIONAL. It appears that it simply lets the Generator know that the
language is optional.  I suggest it should also set a CMake variable that 
CMakeTestCCompiler.cmake (and also CMakeTestCXXCompiler.cmake and
CMakeTestFortranCompiler.cmake) could use to send STATUS or FATAL_ERROR
messages as appropriate.

The reason I am bringing up these OPTIONAL C, C++, and Fortran language
support issues now is I am beginning to encounter reports from PLplot users
with broken compilers, and I would prefer to give them a much smoother
landing (disable that part of PLplot that depends on the compiler in
question with a warning message and keep going) than is possible now.  Once
the OPTIONAL signature to enable_language actually works, I presume a 
substantial number of other multi-language projects would find it useful as
well.

Alan
__________________________
Alan W. Irwin

Astronomical research affiliation with Department of Physics and Astronomy,
University of Victoria (astrowww.phys.uvic.ca).

Programming affiliations with the FreeEOS equation-of-state implementation
for stellar interiors (freeeos.sf.net); PLplot scientific plotting software
package (plplot.org); the libLASi project (unifont.org/lasi); the Loads of
Linux Links project (loll.sf.net); and the Linux Brochure Project
(lbproject.sf.net).
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Linux-powered Science
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