Cannot build on Windows for ARM64: error on init_priority attribute.

[carlo-bramini]

I tried to compile Boost libraries with aarch64-w64-mingw32 cross compiler.
Unfortunately, the build process failed after a while with this error message:

boost-1.90.0-1.noarch/src/boost-1.90.0/libs/nowide/src/iostream.cpp:148:70: error: 'init_priority' attribute ignored [-Werror=attributes]
  148 | #define BOOST_NOWIDE_INIT_PRIORITY __attribute__((init_priority(101)))
      |                                                                      ^

At the time of writing, the support for init_priority is tested into two CMakeLists.txt files:

• libs/filesystem/CMakeLists.txt
• libs/nowide/CMakeLists.txt

Inside these scripts, check_cxx_source_compiles() is used.
These calls check if the test compiles fine and they define BOOST_NOWIDE_HAS_INIT_PRIORITY and BOOST_FILESYSTEM_HAS_INIT_PRIORITY if everything looks ok.
However, these tests do not fail if there is a just warning.
For example, I tried to compile the test source myself and I got this:

$ aarch64-w64-mingw32-g++ boost-1.90.0-1.noarch/src/boost-1.90.0/libs/nowide/config/check_attribute_init_priority.cpp -c -o c.o
boost-1.90.0-1.noarch/src/boost-1.90.0/libs/nowide/config/check_attribute_init_priority.cpp:9:43: warning: 'init_priority' attribute ignored [-Wattributes]
    9 | Foo foo __attribute__((init_priority(101)));
      |                                           ^

So, for CMake the test was good anyways.
The problem comes out because the -Werror option, which makes the build process to fail later.

I have not enough knowledge at the moment, but I can see these two solutions:
Adding -Wno-error=attributes (if supported) when building those libraries, or
Adding -Werror (if supported) when compiling the tests with check_cxx_source_compiles().

This issue happens when building for Windows on ARM64 with GCC.
The i686-w64-mingw32 and x86_64-w64-mingw32 cross compilers also work with that attribute, but they don't emit any warning and that's why this issue was not seen before.

I hope that you will find this report useful.

EDIT: it looks like these scripts already handle CMAKE_REQUIRED_FLAGS before calling check_cxx_source_compiles():

• libs/atomic/CMakeLists.txt
• libs/log/CMakeLists.txt

Probably, it would be enough to follow the same way for adding the needed option.