[Bug libstdc++/88738] treat shared_ptr and unique_ptr more like plain old pointers

[drepper.fsp+rhbz at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88738

--- Comment #3 from Ulrich Drepper <drepper.fsp+rhbz at gmail dot com> ---
Created attachment 45416
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=45416&action=edit
Add nodiscard support

As Martin suggested, we could indeed use existing attributes in library code to
warn about some of the problems.  The code from comment #0 is real, this
happened in a project of mine where I mistyped an assignment.  The warning
would have pointed to the problem.

How about the following patch for a start?  This compiles cleanly on x86-64.  I
haven't run the test suite to see whether it breaks some regression tests.

Also, this approach should be extended beyond shared_ptr and unique_ptr,
probably to at least every single bool operatorXX(...) const.  Or even every
single const member function which then of course raises the question whether
the compiler should learn about this…