The C++ mangler conflates mangling for unsigned, unsigned long, and unsigned long long depending on the target architecture. Those are separate manglings in C++, independent of their target size.
This leads to several linking problems w/ C++ code, in particular when a size_t parameter is involved.
We already have some places in cppmangle where we properly handle Id.__c_long/Id.__c_ulong. This should be extended to size_t parameters in extern(C++) functions. We might also consider to change the size_t declaration to exactly match the C++ definition of the used platform.