[Bug c++/109642] False Positive -Wdangling-reference with std::span-like classes

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109642

--- Comment #19 from GCC Commits <cvs-commit at gcc dot gnu.org> ---
The trunk branch has been updated by Marek Polacek <mpolacek@gcc.gnu.org>:

https://gcc.gnu.org/g:c7607c4cf18986025430ca8626abfe56bfe87106

commit r14-9263-gc7607c4cf18986025430ca8626abfe56bfe87106
Author: Marek Polacek <polacek@redhat.com>
Date:   Thu Jan 25 16:38:51 2024 -0500

    c++: implement [[gnu::no_dangling]] [PR110358]

    Since -Wdangling-reference has false positives that can't be
    prevented, we should offer an easy way to suppress the warning.
    Currently, that is only possible by using a #pragma, either around the
    enclosing class or around the call site.  But #pragma GCC diagnostic tend
    to be onerous.  A better solution would be to have an attribute.

    To that end, this patch adds a new attribute, [[gnu::no_dangling]].
    This attribute takes an optional bool argument to support cases like:

      template <typename T>
      struct [[gnu::no_dangling(std::is_reference_v<T>)]] S {
         // ...
      };

            PR c++/110358
            PR c++/109642

    gcc/cp/ChangeLog:

            * call.cc (no_dangling_p): New.
            (reference_like_class_p): Use it.
            (do_warn_dangling_reference): Use it.  Don't warn when the function
            or its enclosing class has attribute gnu::no_dangling.
            * tree.cc (cxx_gnu_attributes): Add gnu::no_dangling.
            (handle_no_dangling_attribute): New.

    gcc/ChangeLog:

            * doc/extend.texi: Document gnu::no_dangling.
            * doc/invoke.texi: Mention that gnu::no_dangling disables
            -Wdangling-reference.

    gcc/testsuite/ChangeLog:

            * g++.dg/ext/attr-no-dangling1.C: New test.
            * g++.dg/ext/attr-no-dangling2.C: New test.
            * g++.dg/ext/attr-no-dangling3.C: New test.
            * g++.dg/ext/attr-no-dangling4.C: New test.
            * g++.dg/ext/attr-no-dangling5.C: New test.
            * g++.dg/ext/attr-no-dangling6.C: New test.
            * g++.dg/ext/attr-no-dangling7.C: New test.
            * g++.dg/ext/attr-no-dangling8.C: New test.
            * g++.dg/ext/attr-no-dangling9.C: New test.