c++: Implement C++26 P0609R3 - Attributes for Structured Bindings [PR114456]

The following patch implements the P0609R3 paper; we build the
VAR_DECLs for the structured binding identifiers early, so all we need
IMHO is just to parse the attributed identifier list and pass the attributes
to the VAR_DECL creation.

The paper mentions maybe_unused and gnu::nonstring attributes as examples
where they can be useful.  Not sure about either of them.
For maybe_unused, the thing is that both GCC and clang already don't
diagnose maybe unused for the structured binding identifiers, because it
would be a false positive too often; and there is no easy way to find out
if a structured binding has been written with the P0609R3 paper in mind or
not (maybe we could turn it on if in the structured binding is any
attribute, even if just [[]] and record that as a flag on the whole
underlying decl, so that we'd diagnose
  auto [a, b, c[[]]] = d;
  // use a, c but not b
but not
  auto [e, f, g] = d;
  // use a, c but not b
).  For gnu::nonstring, the issue is that we currently don't allow the
attribute on references to char * or references to char[], just on
char */char[].  I've filed a PR for that.

The first testcase in the patch tests it on [[]] and [[maybe_unused]],
just whether it is parsed properly, second on gnu::deprecated, which
works.  Haven't used deprecated attribute because the paper said that
attribute is for further investigation.

2024-04-30  Jakub Jelinek  <jakub@redhat.com>

	PR c++/114456
gcc/c-family/
	* c-cppbuiltin.cc (c_cpp_builtins): Predefine
	__cpp_structured_bindings for C++26 to 202403L rather than
	201606L.
gcc/cp/
	* parser.cc (cp_parser_decomposition_declaration): Implement C++26
	P0609R3 - Attributes for Structured Bindings.  Parse attributed
	identifier lists for structured binding declarations, pass the
	attributes to start_decl.
gcc/testsuite/
	* g++.dg/cpp26/decomp1.C: New test.
	* g++.dg/cpp26/decomp2.C: New test.
	* g++.dg/cpp26/feat-cxx26.C (__cpp_structured_bindings): Expect
	202403 rather than 201606.

0 files changed, 0 insertions, 0 deletions