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Created attachment 58100 [details]
gcc14-pr114459.patch

Untested implementation.

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

https://gcc.gnu.org/g:17458d2bc74b904968e6bdc12527eb040c8d2370

commit r15-304-g17458d2bc74b904968e6bdc12527eb040c8d2370
Author: Jakub Jelinek <jakub@redhat.com>
Date:   Tue May 7 22:38:01 2024 +0200

    c++: Implement C++26 P2893R3 - Variadic friends [PR114459]
    
    The following patch imeplements the C++26 P2893R3 - Variadic friends
    paper.  The paper allows for the friend type declarations to specify
    more than one friend type specifier and allows to specify ... at
    the end of each.  The patch doesn't introduce tentative parsing of
    friend-type-declaration non-terminal, but rather just extends existing
    parsing where it is a friend declaration which ends with ; after the
    declaration specifiers to the cases where it ends with ...; or , or ...,
    In that case it pedwarns for cxx_dialect < cxx26, handles the ... and
    if there is , continues in a loop to parse the further friend type
    specifiers.
    
    2024-05-07  Jakub Jelinek  <jakub@redhat.com>
    
            PR c++/114459
    gcc/c-family/
            * c-cppbuiltin.cc (c_cpp_builtins): Predefine
            __cpp_variadic_friend=202403L for C++26.
    gcc/cp/
            * parser.cc (cp_parser_member_declaration): Implement C++26
            P2893R3 - Variadic friends.  Parse friend type declarations
            with ... or with more than one friend type specifier.
            * friend.cc (make_friend_class): Allow TYPE_PACK_EXPANSION.
            * pt.cc (instantiate_class_template): Handle PACK_EXPANSION_P
            in friend classes.
    gcc/testsuite/
            * g++.dg/cpp26/feat-cxx26.C (__cpp_variadic_friend): Add test.
            * g++.dg/cpp26/variadic-friend1.C: New test.

Implemented for 15.1.