[Bug libstdc++/108846] New: std::copy, std::copy_n on potentially overlapping subobjects

["arthur.j.odwyer at gmail dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108846

            Bug ID: 108846
           Summary: std::copy, std::copy_n on potentially overlapping
                    subobjects
           Product: gcc
           Version: 13.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: arthur.j.odwyer at gmail dot com
  Target Milestone: ---

// https://gcc.godbolt.org/z/EfdG4nzv9
#include <algorithm>
#include <cassert>
struct B { 
    B(int i, short j) : i(i), j(j) {}
    int i; 
    short j; /* 2 byte padding */ 
};
struct D : B { 
    D(int i, short j, short x) : B(i, j), x(x) {}    
    short x;
};
int main() {
    D ddst(1, 2, 3);
    D dsrc(4, 5, 6);
    B *dst = &ddst;
    B *src = &dsrc;
    std::copy_n(src, 1, dst);  // should do `*dst = *src`
    assert(ddst.x == 3);  // FAILS
}

Similarly if you use `std::copy(src, src+1, dst)`.
The problem here is that `std::copy` and `std::copy_n` (and presumably some
other algorithms too, like `std::move`) believe that if a type is trivially
copyable, then it's safe to use `memcpy` or `memmove` on it. But in fact that's
safe only if either
- the type's "sizeof" is equal to its "data size, without trailing padding"; or
- you happen to have external information proving that the object being copied
is not a potentially overlapping subobject.

I think it's safe for `std::copy` and `std::copy_n` to use memcpy/memmove if
they are given a destination range of 2-or-more elements; but if it's just a
single object, then (by the letter of the law) they must assume the destination
object might be a potentially overlapping subobject, and not memcpy into it.

(Full disclosure: I will be THRILLED if you close this as "not a bug," because
that will be ammunition to go to LWG and say "look, vendors think this behavior
is fine, we should actually standardize this behavior and remove the
expectation that STL algorithms can ever handle potentially overlapping
subobjects.")