has_function_attribute() depends on -Wattributes being emitted when an attribute
is not supported by the compiler. In case of GCC on Window (at least) there is no
warning in case the attribute is used on a declaration. Only once there is also a
function definition does it emit a warning like:
a.c: In function ‘foo’:
a.c:8:1: warning: visibility attribute not supported in this configuration; ignored [-Wattributes]
8 | }
To fix this add a dummy function definition to all visibility compiler checks in meson.
The tests in "197 function attributes" only checked for positive return result on on-msvc
compilers, except one special case for dllexport/dllimport. Refactor the tests a bit so
one can specify also a negative expected result, and add tests for all visibility attribute
variants.
* c_function_attributes: remove 'protected' from 'visibility'
This doesn't exist on macos with the apple compiler, which always causes
failures.
Fixes#5530
* compilers: Add split visibility checks to has_function_attribute
These check for a single visibility at a time, rather than all four at
once. This allows for finer grained searches, and should make using
these checks safer across operating systems.
It's fairly common on Linux and *BSD platforms to check for these
attributes existence, so it makes sense to me to have this checking
build into meson itself. Autotools also has a builtin for handling
these, and by building them in we can short circuit cases that we know
that these don't exist (MSVC).
Additionally this adds support for two common MSVC __declspec
attributes, dllimport and dllexport. This implements the declspec
version (even though GCC has an __attribute__ version that both it and
clang support), since GCC and Clang support the MSVC version as well.
Thus it seems reasonable to assume that most projects will use the
__declspec version over teh __attribute__ version.
Christoph Reiter