Bug 114456

I'll probably take this for stage1.

.

Created attachment 58045 [details]
gcc15-pr114456.patch

Untested implementation.

The master branch has been updated by Jakub Jelinek <jakub@gcc.gnu.org>:

https://gcc.gnu.org/g:bd35a92f8d44e91c96e8b6f01805fe4a68acf9eb

commit r15-61-gbd35a92f8d44e91c96e8b6f01805fe4a68acf9eb
Author: Jakub Jelinek <jakub@redhat.com>
Date:   Tue Apr 30 08:57:15 2024 +0200

    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.

Implemented for GCC 15.