[libcxx-commits] [libcxx] Add `_LIBCPP_PLACEMENT_NEW_DEFINED` macro to allow for user defined placement new operators (PR #70538)

[Louis Dionne via libcxx-commits]

ldionne wrote:

> #### Just include `<new>` and stop working around the `std`
> Include times. `<new>` is expensive to include on a variety of `std` implementations.

I just looked and our `<new>` header should be really cheap to include. Other stdlibs should also make that cheap, otherwise it's arguably a QOI defect against them. Another option is to use `#include <version>` and then check for `#ifdef _LIBCPP_VERSION` to check whether libc++ is in use or not -- then you can include `<new>` only if libc++ is in use cause it's cheap to do so. On other implementations that are more expensive you can use another workaround such as `__PLACEMENT_NEW_INLINE`.

I am quite reluctant to add any kind of complexity to the library to work around something like this -- I don't think that's the right tradeoff here.

https://github.com/llvm/llvm-project/pull/70538