[libcxx] std::get_new_handler is not declared when building for VC runtime

[FatihBAKIR]

new_handler.h for libc++ is currently implemented as the following when building for the VC runtime, and not declaring any other symbol, where the new.h resolves to ucrt/new.h in the Windows SDK.

#if defined(_LIBCPP_ABI_VCRUNTIME)
#  include <new.h>
#else

However, new.h does not declare std::get_new_handler, and builds of programs using std::get_new_handler fail. I run into this when building mimalloc for instance.

Is this a known issue? Adding the declaration to new_handler.h fixes the build problems and all mimalloc tests pass. Are there any concerns with such a fix?

[mstorsjo]

This is a known issue. Hacking around it probably is easy, but doing a proper fix seems to be not entirely trivial.

I have a faint memory that there is a fix attempt PR which is in review, but I don't remember the status of it - feel free to search for it and link it here. There was an earlier attempt on https://reviews.llvm.org as well - I think that one is linked from the current PR in review too.

[frederick-vs-ja]

See #150182. Note that MSVC STL doesn't include <new.h> and manually declares get_new_handler. I wonder whether we should do the same.