[Bug c++/125989] [C++29] P2287R6, Designated-initializers for Base Classes

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=125989

--- Comment #4 from GCC Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Jakub Jelinek <[email protected]>:

https://gcc.gnu.org/g:d609ca7423dc3bae4b971a16303129d95512acac

commit r17-3010-gd609ca7423dc3bae4b971a16303129d95512acac
Author: Jakub Jelinek <[email protected]>
Date:   Thu Aug 6 10:51:44 2026 +0200

    c++: implement C++29 P2287R6 - Designated-initializers for Base Classes
[PR125989]

    The following patch implements the C++29 P2287R6
    Designated-initializers for Base Classes
    paper.
    One change is during parsing, to match the new designated-initializer-list
    grammar, the patch allows non-designated clauses followed by designated
    clauses for C++29 or for C++20 to C++26 as pedwarned extension (but rejects
    designated followed by non-designated and also when using GNU style
    [0] = designators non-designated by designated with that GNU array style,
    while the grammar allows now in theory
    { 1, 2, [3] = 3, [4] = 4 } in theory I think it is undesirable to allow
    that mixing, the designated after non-designated has been added solely
    for base classes and arrays don't have those).
    Another change is in the if (first_desig && cxx_dialect >= cxx20) hunk,
    to reject non-designated clause which doesn't appertain to base class
    followed by designated clause.
    Yet another change is the lookup_member as fallback to get_class_binding,
    but according to the paper it shouldn't replace it, even if there is
    ambiguity, if the designator names a NSDM of current class, that should
    be what is used, only if it is an ambiguity inside of base classes there
    should be an error.
    And finally, if field for C++29 is from some base class, find out the
    corresponding base FIELD_DECL and find out how many consecutive designators
    belong to the same FIELD_DECL and recurse using reshape_init_r.

    2026-08-06  Jakub Jelinek  <[email protected]>
                Jason Merrill  <[email protected]>

            PR c++/125989
    gcc/c-family/
            * c-cppbuiltin.cc (c_cpp_builtins): Predefine
            __cpp_designated_initializers to 202606 instead of 201707 for
            C++29.
    gcc/cp/
            * parser.cc: Implement C++29 P2287R6 - Designated-initializers for
            Base Classes.
            (cp_parser_initializer_list): For C++29, allow non-designated
clauses
            followed by designated ones, provided designators are identifiers.
            For C++20-26, allow that as a pedwarned about extension but use the
            C++20 wording in that case.
            * decl.cc (reshape_init_class): Diagnose non-designated clause not
            appertaining to base class followed by designated clause for C++ >=
20.
            For C++29, if get_class_binding for designator is unsuccessful, use
            lookup_member afterwards and handle the case where the lookup
succeeds
            for some member of a base class through recursive
reshape_init_class.
    gcc/testsuite/
            * g++.dg/cpp/embed-14.C: Expect different diagnostics for C++29 and
            sometimes for C++20-C++26 too.
            * g++.dg/cpp2a/desig2.C: Likewise.
            * g++.dg/cpp2a/desig13.C: Likewise.
            * g++.dg/cpp2a/desig20.C: Likewise.
            * g++.dg/ext/desig4.C: Likewise.
            * g++.dg/parse/pr43765.C: Likewise.
            * g++.dg/cpp29/feat-cxx29.C: Expect different value of
            __cpp_designated_initializers for C++29.
            * g++.dg/cpp29/desig1.C: New test.
            * g++.dg/cpp29/desig2.C: New test.
            * g++.dg/cpp29/desig3.C: New test.
            * g++.dg/cpp29/desig4.C: New test.
            * g++.dg/cpp29/desig5.C: New test.
            * g++.dg/cpp29/desig6.C: New test.
            * g++.dg/cpp29/desig7.C: New test.
            * g++.dg/cpp29/desig8.C: New test.
            * g++.dg/cpp29/desig9.C: New test.
            * g++.dg/cpp29/desig10.C: New test.
            * g++.dg/cpp29/desig11.C: New test.
            * g++.dg/cpp29/desig12.C: New test.
            * g++.dg/cpp29/desig13.C: New test.
            * g++.dg/cpp29/desig14.C: New test.
            * g++.dg/cpp29/desig15.C: New test.
            * g++.dg/cpp29/desig16.C: New test.

    Reviewed-by: Jason Merrill <[email protected]>