[Bug]: ABSL_DECLARE_FLAG can't work with MSVC __declspec user MACRO

[Mizux]

Describe the issue

When build a shared lib on windows you usually need a dllspec macro (like ABSL_CONSUME_DLL/ABSL_DLL), unfortunately the current MACRO implem of ABSL_DECLARE_FLAG() do not support this use case...

e.g. if we try to prepend the MACRO...
foo.h

#include "absl/flags/declare.h"
...
#if defined(_MSC_VER) && defined(FOO_BUILD_DLL)
// Annoying stuff for windows -- makes sure clients can import these functions
#if defined(FOO_EXPORT)
#define FOO_DLL __declspec(dllexport)
#else
#define FOO_DLL __declspec(dllimport)
#endif  // defined(FOO_EXPORT)
#endif  // _MSC_VER && FOO_BUILD_DLL

#ifndef FOO_DLL
#define FOO_DLL
#endif

FOO_DLL ABSL_DECLARE_FLAG(bool, foo_bool);
FOO_DLL ABSL_DECLARE_FLAG(int, foo_int);
FOO_DLL ABSL_DECLARE_FLAG(int64_t, foo_long_int);
FOO_DLL ABSL_DECLARE_FLAG(std::string, foo_string);

with CMakeLists.txt:

set(BUILD_SHARED_LIBS ON)
...
add_library(foo)
...
if(MSVC AND BUILD_SHARED_LIBS)
  target_compile_definitions(foo PUBLIC "FOO_BUILD_DLL")
  target_compile_definitions(foo PRIVATE "FOO_EXPORT")
endif()

and later in an app.exe's CMakeLists.txt

add_executable(app)
...
target_link_libraries(app PRIVATE foo)

Since the ABSL_DECLARE_FLAG_INTERNAL implementation declares two time the flags (line 63 and 66)

abseil-cpp/absl/flags/declare.h

Lines 62 to 66 in 0f102ad

	#define ABSL_DECLARE_FLAG_INTERNAL(type, name) \
	extern absl::Flag<type> FLAGS_##name; \
	namespace absl /* block flags in namespaces */ {} \
	/* second redeclaration is to allow applying attributes */ \
	extern absl::Flag<type> FLAGS_##name

we can't prepand with our FOO_DLL nor we can provide it to the macro...

This regression aka double declaration has been introduced in:
1065514#diff-252f54025d208b3afe13aa8d62762bb88282ad00027be08396105994d31f813f

Steps to reproduce the problem

You have a complete working/borken example here. just test on windows with a cmake based build and play with the #if 0/1 to enable my custom macro fix or show the issue...
relevant src: https://github.com/Mizux/absl-cxx/blob/715fdb721515248b8b8c047325f78d46f7dfc0c8/foo/foo.hpp#L25-L41

On any windows git bash with MSVC and CMake installed.

git clone ... && cd absl-cxx
# optional: to check the generator is MSVC 2022
cmake -G 
cmake -S. -Bbuild
cmake --build build --config Release --target app

Expected: compile + link ok

Observed:
if using FOO_DLL ABSL_DECLARE_FLAG(...) path

C:\Users\mizux\dev\absl-cxx\foo\foo.hpp(43,9): warning C4273: 'FLAGS_foo_bool': inconsistent dll linkage [C:\Users\mizux\dev\absl-cxx\build\app\app.vcxproj]
  (compiling source file '../../app/app.cpp')
      C:\Users\mizux\dev\absl-cxx\foo\foo.hpp(43,9):
      see previous definition of 'FLAGS_foo_bool'
...
  foo.vcxproj -> C:\Users\mizux\dev\absl-cxx\build\Release\bin\foo.dll
app.obj : error LNK2019: unresolved external symbol "class absl::lts_20240722::flags_internal::Flag<bool> FLAGS_foo_bool" (?FLAGS_foo_bool@@3V?$Flag@_N@flags_internal@lts_20240722@absl@@A) referenced in function main [C:\Users\mizux\dev\absl-cxx\build\app\app.vcxproj]
app.obj : error LNK2019: unresolved external symbol "class absl::lts_20240722::flags_internal::Flag<int> FLAGS_foo_int" (?FLAGS_foo_int@@3V?$Flag@H@flags_internal@lts_20240722@absl@@A) referenced in function main [C:\Users\mizux\dev\absl-cxx\build\app\app.vcxproj]
app.obj : error LNK2019: unresolved external symbol "class absl::lts_20240722::flags_internal::Flag<__int64> FLAGS_foo_long_int" (?FLAGS_foo_long_int@@3V?$Flag@_J@flags_internal@lts_20240722@absl@@A) referenced in function main [C:\Users\mizux\dev\absl-cxx\build\app\app.vcxproj]
app.obj : error LNK2019: unresolved external symbol "class absl::lts_20240722::flags_internal::Flag<class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> > > FLAGS_foo_string" (?FLAGS_foo_string@@3V?$Flag@V?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@std@@@flags_internal@lts_20240722@absl@@A) referenced in function main [C:\Users\mizux\dev\absl-cxx\build\app\app.vcxproj]
C:\Users\mizux\dev\absl-cxx\build\Release\bin\app.exe : fatal error LNK1120: 4 unresolved externals [C:\Users\mizux\dev\absl-cxx\build\app\app.vcxproj]

What version of Abseil are you using?

20240722.0 with a small patch to check c++ dialect detected IIRC

patch: https://github.com/Mizux/absl-cxx/blob/main/patches/abseil-cpp-20240722.0.patch
note: IIRC the patch was for debian where while we want to compile in C++17 absl figure out that the default compiler support c++20 and do not respect the CMAKE_CXX_STANDARD set before the FetchContent/MakeAvailable. Also a CMake version got a regression where check_cxx_source_compiles didn't capture the CMAKE_CXX_STANDARD....
EDIT: seems related to https://gitlab.kitware.com/cmake/cmake/-/merge_requests/306

ref: https://github.com/Mizux/absl-cxx/blob/715fdb721515248b8b8c047325f78d46f7dfc0c8/cmake/dependencies/CMakeLists.txt#L40-L42

What operating system and version are you using?

MSVC Community 2022 (dllspec issue) gWindows cloudtop and windows 11 (X1 from re-stuff)

What compiler and version are you using?

msvc

What build system are you using?

CMake 3.29.5 (from VS Community 2022)

Additional context

the current workaround we use for google/or-tools is to define new MACRO

// The ABSL_DECLARE_DLL_FLAG(dll, type, name) macro expands to:
//   dll extern absl::Flag<type> FLAGS_name;
#define ABSL_DECLARE_DLL_FLAG(dll, type, name) ABSL_DECLARE_DLL_FLAG_INTERNAL(dll, type, name)

// Internal implementation of ABSL_DECLARE_FLAG to allow macro expansion of its
// arguments. Clients must use ABSL_DECLARE_FLAG instead.
#define ABSL_DECLARE_DLL_FLAG_INTERNAL(dll, type, name)      \
  dll extern absl::Flag<type> FLAGS_##name;                  \
  namespace absl /* block flags in namespaces */ {}          \
  /* second redeclaration is to allow applying attributes */ \
  dll extern absl::Flag<type> FLAGS_##name

usage:

ABSL_DECLARE_DLL_FLAG(FOO_DLL, bool, foo_bool);
ABSL_DECLARE_DLL_FLAG(FOO_DLL, int, foo_int);
ABSL_DECLARE_DLL_FLAG(FOO_DLL, int64_t, foo_long_int);
ABSL_DECLARE_DLL_FLAG(FOO_DLL, std::string, foo_string);

=> Would it be possible to add it to abseil-cpp directly ?

[Mizux]

An other suggestion is to change the implementation of ABSL_DECLARE_FLAG_INTERNAL on windows:

// Internal implementation of ABSL_DECLARE_FLAG to allow macro expansion of its
// arguments. Clients must use ABSL_DECLARE_FLAG instead.
#if defined(_MSC_VER)
  #define ABSL_DECLARE_FLAG_INTERNAL(type, name) \
    extern absl::Flag<type> FLAGS_##name
#else
  #define ABSL_DECLARE_DLL_FLAG_INTERNAL(type, name)      \
    extern absl::Flag<type> FLAGS_##name;                  \
    namespace absl /* block flags in namespaces */ {}          \
    /* second redeclaration is to allow applying attributes */ \
    extern absl::Flag<type> FLAGS_##name
#endif  // _MSC_VER

Here a CL to apply this fix in google3 in //thirdparty... cl/722558570
ref: https://opensource.google/documentation/reference/thirdparty

note: This fix is needed to properly fix:

• [Bug]: cmake: Can't access to any gtest's flags when compiling with BUILD_SHARED_LIBS=ON and GTEST_HAS_ABSL=ON google/googletest#4718