Patch: https://gcc.gnu.org/pipermail/gcc-patches/2022-November/605705.html

https://gcc.gnu.org/g:6492cec069bccf817ac5e984fb3eca407056a566

commit r13-4046-g6492cec069bccf817ac5e984fb3eca407056a566
Author: Jakub Jelinek <jakub@redhat.com>
Date:   Tue Nov 15 08:00:21 2022 +0100

    c++: Implement C++23 P2589R1 - - static operator[]
    
    Here is a patch that implements the static operator[] paper.
    One thing that doesn't work properly is the same problem as I've filed
    yesterday for static operator() - PR107624 - that side-effects of
    the postfix-expression on which the call or subscript operator are
    applied are thrown away, I assume we have to add them into COMPOUND_EXPR
    somewhere after we find out that the we've chosen a static member function
    operator.
    
    2022-11-15  Jakub Jelinek  <jakub@redhat.com>
    
    gcc/c-family/
            * c-cppbuiltin.cc (c_cpp_builtins): Bump C++23
            __cpp_multidimensional_subscript macro value to 202211L.
    gcc/cp/
            * decl.cc (grok_op_properties): Implement C++23 P2589R1
            - static operator[].  Handle operator[] similarly to operator()
            - allow static member functions, but pedwarn on it for C++20 and
            older.  Unlike operator(), perform rest of checks on it though for
            C++20.
            * call.cc (add_operator_candidates): For operator[] with class
            typed first parameter, pass that parameter as first_arg and
            an adjusted arglist without that parameter.
    gcc/testsuite/
            * g++.dg/cpp23/subscript9.C: New test.
            * g++.dg/cpp23/feat-cxx2b.C: Expect a newer
            __cpp_multidimensional_subscript value.
            * g++.old-deja/g++.bugs/900210_10.C: Don't expect an error
            for C++23 or later.