c++: Implement part of C++26 P2686R4 - constexpr structured bindings [PR117784]

The following patch implements the constexpr structured bindings part of
the P2686R4 paper, so the [dcl.pre], [dcl.struct.bind], [dcl.constinit]
and first hunk in [dcl.constexpr] changes.
The paper doesn't have a feature test macro and the constexpr structured
binding part of it seems more-less self-contained, so I think it is useful
to get this in independently from the rest.
Of course, automatic constexpr/constinit structured bindings in the
tuple cases or automatic constexpr/constinit structured bindings with auto &
will not really work for now.
Another reason for the split is that for C++ < 26, I think what the patch
implements is basically what the users will see, i.e. we can accept
constexpr or constinit structured binding with pedwarn, but I think we can't
change the constant expression rules in C++ < 26.

I plan to look at the rest of the paper.

2025-07-08  Jakub Jelinek  <jakub@redhat.com>

	PR c++/117784
	* decl.cc: Implement part of C++26 P2686R4 - constexpr structured
	bindings.
	(cp_finish_decl): Pedwarn for C++23 and older on constinit on
	structured bindings except for static/thread_local where it uses
	earlier error.
	(grokdeclarator): Pedwarn on constexpr structured bindings for
	C++23 and older instead of emitting error always, don't clear
	constexpr_p in that case.
	* parser.cc (cp_parser_decomposition_declaration): Copy over
	DECL_DECLARED_CONSTEXPR_P and DECL_DECLARED_CONSTINIT_P flags.

	* g++.dg/cpp1z/decomp3.C (test): For constexpr structured binding
	initialize from constexpr var instead of non-constexpr and expect
	just a pedwarn for C++23 and older instead of error always.
	* g++.dg/cpp26/decomp9.C (foo): Likewise.
	* g++.dg/cpp26/decomp22.C: New test.
	* g++.dg/cpp26/decomp23.C: New test.
	* g++.dg/cpp26/decomp24.C: New test.
	* g++.dg/cpp26/decomp25.C: New test.

0 files changed, 0 insertions, 0 deletions