c++: Implement for static locals CWG 2867 - Order of initialization for structured...

c++: Implement for static locals CWG 2867 - Order of initialization for structured bindings [PR115769]

On Wed, Aug 14, 2024 at 10:06:24AM +0200, Jakub Jelinek wrote:
> Though, now that I think about it again, perhaps what we could do instead
> is just make sure the _ZGVZ3barvEDC1x1y1z1wE initialization doesn't have
> a CLEANUP_POINT_EXPR in it and wrap both the _ZGVZ3barvEDC1x1y1z1wE
> and cp_finish_decomp created stuff into a single CLEANUP_POINT_EXPR.
> That way, perhaps _ZGVZ3barvEDC1x1y1z1wE could be initialized by one thread
> and _ZGVZ3barvE1x by a different, but the temporaries from _ZGVZ3barvEDC1x1y1z1wE
> initialization would be only destructed after the _ZGVZ3barvE1w guard
> was released by the thread which initialized _ZGVZ3barvEDC1x1y1z1wE.

Here is the I believe ABI compatible version, which uses the separate
guard variables, so different structured binding variables can be
initialized in different threads, but the thread that did the artificial
base initialization will keep temporaries live at least until the last
guard variable is released (i.e. when even that variable has been
initialized).

2025-01-22  Jakub Jelinek  <jakub@redhat.com>

	PR c++/115769
	* decl.cc: Partially implement CWG 2867 - Order of initialization
	for structured bindings.
	(cp_finish_decl): If need_decomp_init, for function scope structure
	binding bases, temporarily clear stmts_are_full_exprs_p before
	calling expand_static_init, after it call cp_finish_decomp and wrap
	code emitted by both into maybe_cleanup_point_expr_void and ensure
	cp_finish_decomp isn't called again.

	* g++.dg/DRs/dr2867-3.C: New test.
	* g++.dg/DRs/dr2867-4.C: New test.