cmake: Incorrect `__float128` detection

[iv-m]

Current develop branch uses the following CMake code to detect presence of __float128 types:

charconv/CMakeLists.txt

Line 23 in 100473b

check_cxx_source_compiles(config/has_float128.cpp QUADMATH_FOUND)

But check_cxx_source_compiles function takes code as the first argument, and, as far as I cant tell, cannot read the source from file. So, it tries to compile config/has_float128.cpp as C++ code, and, of course, fails. I'm getting the following error in CMakeFiles/CMakeConfigureLog.yaml:

Run Build Command(s): /usr/bin/cmake -E env VERBOSE=1 /usr/bin/gmake -f Makefile cmTC_38738/fast
/usr/bin/gmake  -f CMakeFiles/cmTC_38738.dir/build.make CMakeFiles/cmTC_38738.dir/build
gmake[1]: Entering directory '/usr/src/boost/bld/CMakeFiles/CMakeScratch/TryCompile-oiusM4'
Building CXX object CMakeFiles/cmTC_38738.dir/src.cxx.o
/usr/bin/c++ -DQUADMATH_FOUND   -o CMakeFiles/cmTC_38738.dir/src.cxx.o -c /usr/src/boost/bld/CMakeFiles/CMakeScratch/TryCompile-oiusM4/src.cxx
/usr/src/boost/bld/CMakeFiles/CMakeScratch/TryCompile-oiusM4/src.cxx:1:1: error: 'config' does not name a type
    1 | config/has_float128.cpp
      | ^~~~~~
gmake[1]: *** [CMakeFiles/cmTC_38738.dir/build.make:78: CMakeFiles/cmTC_38738.dir/src.cxx.o] Error 1
gmake[1]: Leaving directory '/usr/src/boost/bld/CMakeFiles/CMakeScratch/TryCompile-oiusM4'
gmake: *** [Makefile:127: cmTC_38738/fast] Error 2

Indeed /usr/src/boost/bld/CMakeFiles/CMakeScratch/TryCompile-oiusM4/src.cxx contains config/has_float128.cpp literally.

I'm using cmake 3.29.1.

[iv-m]

I think something like this should be better match for what is written in Jamfile

try_compile(QUADMATH_FOUND
            SOURCES ${CMAKE_CURRENT_SOURCE_DIR}/config/has_float128.cpp
            LINK_LIBRARIES quadmath)

[mborland]

I do like the try_compile option, but since it requires CMake 3.25 I think it's far too new for a C++11 library. I'll try pasting in the code from config/has_float128.cpp.

[pdimov]

try_compile doesn't require 3.25, only the newer signatures do.

[mborland]

try_compile doesn't require 3.25, only the newer signatures do.

My mistake. I still think check_cxx_source_runs is preferable here because systems like Alpine linux provide the header <quadmath.h> but not the compiled library. See: #176

[pdimov]

Yes of course. I'm not saying this needs to be changed.

But you should know that check_cxx_source_compiles and check_cxx_source_runs are just wrappers over try_compile and try_run.

https://github.com/Kitware/CMake/blob/master/Modules/CheckCXXSourceCompiles.cmake
https://github.com/Kitware/CMake/blob/master/Modules/Internal/CheckSourceCompiles.cmake

https://github.com/Kitware/CMake/blob/master/Modules/CheckCXXSourceRuns.cmake
https://github.com/Kitware/CMake/blob/master/Modules/Internal/CheckSourceRuns.cmake