Party based on this thread: http://lists.llvm.org/pipermail/cfe-dev/2020-February/064754.html.
This patch merges two of CXXAllocatorCall's parameters, so that we are able to supply a CallEvent object to check::NewAllocatorCall (see the description of D75430 to see why this would be great).
One of the things mentioned by @NoQ was the following:
I think at this point we might actually do a good job sorting out this check::NewAllocator issue because we have a "separate" "Environment" to hold the other SVal, which is "objects under construction"! - so we should probably simply teach CXXAllocatorCall to extract the value from the objects-under-construction trait of the program state and we're good.
I had MallocChecker in my crosshair for now, so I admittedly threw together something as a proof of concept. Now that I know that this effort is worth pursuing though, I'll happily look for a solution better then demonstrated in this patch.