c++: Fix up constexpr evaluation of new with zero sized types [PR104568]

c++: Fix up constexpr evaluation of new with zero sized types [PR104568]

The new expression constant expression evaluation right now tries to
deduce how many elts the array it uses for the heap or heap [] vars
should have (or how many elts should its trailing array have if it has
cookie at the start).  As new is lowered at that point to
(some_type *) ::operator new (size)
or so, it computes it by subtracting cookie size if any from size, then
divides the result by sizeof (some_type).
This works fine for most types, except when sizeof (some_type) is 0,
then we divide by zero; size is then equal to cookie_size (or if there
is no cookie, to 0).
The following patch special cases those cases so that we don't divide
by zero and also recover the original outer_nelts from the expression
by forcing the size not to be folded in that case but be explicit
0 * outer_nelts or cookie_size + 0 * outer_nelts.

Note, we have further issues, we accept-invalid various cases, for both
zero sized elt_type and even non-zero sized elts, we aren't able to
diagnose out of bounds POINTER_PLUS_EXPR like:
constexpr bool
foo ()
{
  auto p = new int[2];
  auto q1 = &p[0];
  auto q2 = &p[1];
  auto q3 = &p[2];
  auto q4 = &p[3];
  delete[] p;
  return true;
}
constexpr bool a = foo ();
That doesn't look like a regression so I think we should resolve that for
GCC 13, but there are 2 problems.  Figure out why
cxx_fold_pointer_plus_expression doesn't deal with the &heap []
etc. cases, and for the zero sized arrays, I think we really need to preserve
whether user wrote an array ref or pointer addition, because in the
&p[3] case if sizeof(p[0]) == 0 we know that if it has 2 elements it is
out of bounds, while if we see p p+ 0 the information if it was
p + 2 or p + 3 in the source is lost.
clang++ seems to handle it fine even in the zero sized cases or with
new expressions.

2022-03-18  Jakub Jelinek  <jakub@redhat.com>

	PR c++/104568
	* init.cc (build_new_constexpr_heap_type): Remove FULL_SIZE
	argument and its handling, instead add ITYPE2 argument.  Only
	support COOKIE_SIZE != NULL.
	(build_new_1): If size is 0, change it to 0 * outer_nelts if
	outer_nelts is non-NULL.  Pass type rather than elt_type to
	maybe_wrap_new_for_constexpr.
	* constexpr.cc (build_new_constexpr_heap_type): New function.
	(cxx_eval_constant_expression) <case CONVERT_EXPR>:
	If elt_size is zero sized type, try to recover outer_nelts from
	the size argument to operator new/new[] and pass that as
	arg_size to build_new_constexpr_heap_type.  Pass ctx,
	non_constant_p and overflow_p to that call too.

	* g++.dg/cpp2a/constexpr-new22.C: New test.