[Bug analyzer/94355] support for C++ new expression

["redi at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94355

--- Comment #9 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to William Navarre from comment #8)
> It seems that `operator new` is generally not supposed to return NULL --
> std::bad_alloc() is supposed to be thrown instead. 

If an operator new overload is declared noexcept, then it can return null on
failure. If it is not noexcept then it throws bad_alloc (or something derived
from it) and must never return null.


> 
> I made that change on my build (see below). I think that treating new's
> result as never-null is probably the correct thing to do most of the time,
> but two considerations: 
> 
> 1) The case of allocating a zero-length array. 

That still can't return null. It must return a valid non-null pointer, that
cannot be derefernced.

> 2) The case that a project has replaced `operator new.` (See "global
> replacements" at https://en.cppreference.com/w/cpp/memory/new/operator_new). 
> 
> Apparently projects can replace `operator new` (see "global replacements" at
> https://en.cppreference.com/w/cpp/memory/new/operator_new). It's not clear 

Exactly the same rules apply. If the operator new function is noexcept it
returns null to indicate allocation failure, otherwise it must throw and cannot
return null, ever.