[Bug libstdc++/116641] New: [12/13/14/15 Regression] std::string move assignment incorrectly depends on POCCA

[redi at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=116641

            Bug ID: 116641
           Summary: [12/13/14/15 Regression] std::string move assignment
                    incorrectly depends on POCCA
           Product: gcc
           Version: 15.0
            Status: UNCONFIRMED
          Keywords: wrong-code
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: redi at gcc dot gnu.org
                CC: oliver.rosten at googlemail dot com
  Target Milestone: ---

Oliver Rosten noticed this bug:

#include <string>

template<typename T>
struct Alloc
{
  using value_type = T;
  using propagate_on_container_swap = std::false_type;
  using propagate_on_container_copy_assignment = std::true_type;
  using propagate_on_container_move_assignment = std::false_type;

  Alloc(int id) : id(id) { }

  template<typename U>
    Alloc(const Alloc<U>& a) : id(a.id) { }

  T* allocate(unsigned long n) { return std::allocator<T>().allocate(n); }
  void deallocate(T* p, unsigned long n) { std::allocator<T>().deallocate(p,
n); }

  Alloc& operator=(const Alloc&) { throw; }

  bool operator==(const Alloc& a) const { return id == a.id; }
  bool operator!=(const Alloc& a) const { return id != a.id; }

  int id;
};


void
test_move_assign()
{
  Alloc<char> a1(1), a2(2);
  std::basic_string<char, std::char_traits<char>, Alloc<char>> s1(a1), s2(a2);

  s1 = "sooooooooooooooooooooooooooooooo long";
  s2 = std::move(s1);
}

int main()
{
  test_move_assign();
}

There should be no allocator assignment here, but since r10-327-gdb33efde17932f
string::assign considers POCCA (propagate_on_container_copy_assignment) and so
when operator=(basic_string&&) calls assign to do a deep copy, it also ends up
propagating the allocator. That shouldn't happen, because POCMA is false here.

The fix should be as simple as changing operator= to call _M_assign instead of
assign.