[Bug libstdc++/103904] [defect fix] Please backport P2325R3 to 10 and 11

["redi at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103904

--- Comment #5 from Jonathan Wakely <redi at gcc dot gnu.org> ---
The issue is whether somebody's code breaks when upgrading from GCC 11.2 to
11.3, or when upgrading from 11.x to 12.x, and the documented policy says the
former should be avoided. https://gcc.gnu.org/develop.html#release says

-->8--
Changes Appropriate for Bug-Fix Releases

As a general rule of thumb, bug-fix releases should contain fixes for
regressions or serious bugs uncovered in the corresponding major release of GCC
or those prior to it. When backporting patches from subsequent releases or from
trunk, care should be taken to avoid making changes that cause previously
accepted code to be rejected unless doing so would cause GCC to generate
incorrect object code or code with undefined behavior.

Rationale

Users have an expectation of upgrading to a bug-fix GCC release without having
to adjust their source code. Avoiding changes that reject code that was
previously accepted makes this possible.
-->8--

The only reason to consider breaking the policy is that C++20 support is
explicitly experimental, so breaking changes are more acceptable.