[Bug c++/107529] New: constexpr evaluator doesn't check for destroyed objects

[vanyacpp at gmail dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107529

            Bug ID: 107529
           Summary: constexpr evaluator doesn't check for destroyed
                    objects
           Product: gcc
           Version: 13.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: vanyacpp at gmail dot com
  Target Milestone: ---

I believe this function contains undefined behavior and should not be allowed
to evaluate at compile-time.

The call to `std::destroy_at(p)` should end the lifetime of `*p` and accesses
to `*p` after that should be invalid.

#include <memory>

struct mytype
{
    constexpr mytype() : x(42) {}
    constexpr ~mytype() {}

    int x;
};

constexpr int foo()
{
    std::allocator<mytype> alloc;
    mytype* p = alloc.allocate(1);
    std::construct_at(p);
    std::destroy_at(p);       // destroy *p
    int result = p->x;        // access
    alloc.deallocate(p, 1);
    return result;
}

static_assert(foo() == 42);