This code compiles cleanly unless you use -Wsystem-headers:

#include <new>
#include <cstdlib>

void* operator new(std::size_t n)
{
  return std::malloc(n);
}

void operator delete(void* p)
{
  std::free(p);
}

void operator delete(void* p, std::size_t)
{
  std::free(p);
}


$ g++ del.C -c
$ g++ del.C -c -Wsystem-headers
del.C:9:6: warning: declaration of ‘void operator delete(void*)’ has a different exception specifier [-Wsystem-headers]
    9 | void operator delete(void* p)
      |      ^~~~~~~~
In file included from del.C:1:
/usr/include/c++/12/new:130:6: note: from previous declaration ‘void operator delete(void*) noexcept’
  130 | void operator delete(void*) _GLIBCXX_USE_NOEXCEPT
      |      ^~~~~~~~
del.C:14:6: warning: declaration of ‘void operator delete(void*, std::size_t)’ has a different exception specifier [-Wsystem-headers]
   14 | void operator delete(void* p, std::size_t)
      |      ^~~~~~~~
/usr/include/c++/12/new:135:6: note: from previous declaration ‘void operator delete(void*, std::size_t) noexcept’
  135 | void operator delete(void*, std::size_t) _GLIBCXX_USE_NOEXCEPT
      |      ^~~~~~~~


But the warning is correct, and indicates a problem in the user code. It should not be suppressed by the system header.