[Bug libstdc++/91357] _GLIBCXX_ASSERTIONS rejects possibly-valid code

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91357

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
More specifically:

        v.operator[](1); /* maybe okay -- forms a pointer one past end */

Not OK. **Dereferences** a past-the end iterator. That's UB. Go to jail. Go
directly to jail. Do not pass Go. Do not collect £200.

        v[1]; /* highly suspicious but not obviously invalid or UB */

UB for the same reason.

        int *ugly_end = &v[1]; /* hmm.  is this okay? */

No. v[1] is UB already, &v[1] doesn't make it unundefined.

        std::cout << v[1] << std::endl;  /* definitely not okay */

        v[2]; /* definitely not okay -- merely computing this pointer is UB */

There's no pointer. vector::operator[] is not specified in terms of pointers.
This is not C.

The assertion is entirely correct. Calling operator[] with an invalid value is
UB, it doesn't matter what you do (or don't do) with the result.