symtab: Fold &a == &b to 0 if folding_initializer [PR94716]

symtab: Fold &a == &b to 0 if folding_initializer [PR94716]

On Thu, Dec 09, 2021 at 06:09:12PM -0500, Jason Merrill wrote:
> For the more general comparison of decls like your a != b example above I
> think clang is in the right; in manifestly constant-evaluated context
> (folding_initializer) we should return that they are unequal and prevent a
> later alias declaration, like we do for comparison to 0 in
> maybe_nonzero_address.  It's possible that this gives a wrong answer based
> on something in another translation unit, but that's unlikely, and taking
> that chance seems better than rejecting code that needs a constant answer.

I agree.  This is an incremental patch to do that.

2022-01-03  Jakub Jelinek  <jakub@redhat.com>

	PR c++/94716
gcc/
	* symtab.c: Include fold-const.h.
	(symtab_node::equal_address_to): If folding_initializer is true,
	handle it like memory_accessed.  Simplify.
gcc/testsuite/
	* gcc.dg/init-compare-1.c: New test.
	* g++.dg/cpp0x/constexpr-compare1.C: New test.
	* g++.dg/cpp1y/constexpr-94716.C: New test.
	* g++.dg/cpp1z/constexpr-compare1.C: New test.