Commits on Feb 13, 2024

1. MSYS2: Export forwarders for __cxa_begin_catch, __cxa_end_catch, … …

   …`__cxa_rethrow` and `__gxx_personality_seh0`
   
   libobjc2 uses native C++ exceptions on MinGW.  The clang compiler will emit references to `__cxa_begin_catch`, `__cxa_end_catch`, `__cxa_rethrow` and `__gxx_personality_seh0` for Objective C code which uses Objective C exceptions.
   
   These symbols are defined in the C++ runtime, not in libobjc2.  As a result, merely linking with libobjc2 is not sufficient.  Objective C code such as GNUstep must be compiled with the `LDFLAGS="-lgcc_s -lstdc++"` or `LDFLAGS="-lc++"`, depending on the environment.
   
   This is tedious.  Additionally, specifying `-lc++` on the msys/clang64 environment causes linker errors:
   
   ```
    Linking library libgnustep-base ...
   ld.lld: error: libc++.dll.a(libc++.dll): .idata$4 should not refer to special section 0
   ```
   
   A [similar error has been observed for other libraries](msys2/MINGW-packages#18589)
   
   A solution for this is to define forwarding exports for `__cxa_begin_catch`, `__cxa_end_catch`, `__cxa_rethrow` and `__gxx_personality_seh0`.  This is implemented by adding a `eh_forwards.def` file to the list of libobjc2 source files, which forwards the symbols to the actual C++ runtime.  On MSYS2, the libstdc++ and libc++ runtimes are supported, which covers all MinGW environments: https://www.msys2.org/docs/environments/.
   
   Forwarding exports are discussed here:
   - https://learn.microsoft.com/en-us/cpp/build/reference/exports?view=msvc-170
   - https://devblogs.microsoft.com/oldnewthing/20060719-24/?p=30473
   - https://devblogs.microsoft.com/oldnewthing/20121116-00/?p=6073

   

   qmfrederik committed Feb 13, 2024
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