https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113798
--- Comment #6 from GCC Commits <cvs-commit at gcc dot gnu.org> --- The trunk branch has been updated by Marek Polacek <mpola...@gcc.gnu.org>: https://gcc.gnu.org/g:c43527216622a119feb0d2ca4c0d8dd37463fe0a commit r15-5758-gc43527216622a119feb0d2ca4c0d8dd37463fe0a Author: Marek Polacek <pola...@redhat.com> Date: Wed Oct 9 11:54:37 2024 -0400 c++: Implement P2662R3, Pack Indexing [PR113798] This patch implements C++26 Pack Indexing, as described in <https://wg21.link/P2662R3>. The issue discussing how to mangle pack indexes has not been resolved yet <https://github.com/itanium-cxx-abi/cxx-abi/issues/175> and I've made no attempt to address it so far. Unlike v1, which used augmented TYPE/EXPR_PACK_EXPANSION codes, this version introduces two new codes: PACK_INDEX_EXPR and PACK_INDEX_TYPE. Both carry two operands: the pack expansion and the index. They are handled in tsubst_pack_index: substitute the index and the pack and then extract the element from the vector (if possible). To handle pack indexing in a decltype or with decltype(auto), there is also the new PACK_INDEX_PARENTHESIZED_P flag. With this feature, it's valid to write something like using U = tmpl<Ts...[Is]...>; where we first expand the template argument into Ts...[Is#0], Ts...[Is#1], ... and then substitute each individual pack index. PR c++/113798 gcc/cp/ChangeLog: * constexpr.cc (potential_constant_expression_1) <case PACK_INDEX_EXPR>: New case. * cp-objcp-common.cc (cp_common_init_ts): Mark PACK_INDEX_TYPE and PACK_INDEX_EXPR. * cp-tree.def (PACK_INDEX_TYPE): New. (PACK_INDEX_EXPR): New. * cp-tree.h (WILDCARD_TYPE_P): Also check PACK_INDEX_TYPE. (PACK_INDEX_CHECK): Define. (PACK_INDEX_P): Define. (PACK_INDEX_PACK): Define. (PACK_INDEX_INDEX): Define. (PACK_INDEX_PARENTHESIZED_P): Define. (make_pack_index): Declare. (pack_index_element): Declare. * cxx-pretty-print.cc (cxx_pretty_printer::expression) <case PACK_INDEX_EXPR>: New case. (cxx_pretty_printer::type_id) <case PACK_INDEX_TYPE>: New case. * error.cc (dump_type) <case PACK_INDEX_TYPE>: New case. (dump_type_prefix): Handle PACK_INDEX_TYPE. (dump_type_suffix): Likewise. (dump_expr) <case PACK_INDEX_EXPR>: New case. * mangle.cc (write_type) <case PACK_INDEX_TYPE>: New case. * module.cc (trees_out::type_node) <case PACK_INDEX_TYPE>: New case. (trees_in::tree_node) <case PACK_INDEX_TYPE>: New case. * parser.cc (cp_parser_next_tokens_are_pack_index_p): New. (cp_parser_pack_index): New. (cp_parser_primary_expression): Handle a C++26 pack-index-expression. (cp_parser_unqualified_id): Handle a C++26 pack-index-specifier. (cp_parser_nested_name_specifier_opt): See if a pack-index-specifier follows. (cp_parser_qualifying_entity): Handle a C++26 pack-index-specifier. (cp_parser_decltype_expr): Set id_expression_or_member_access_p for pack indexing. (cp_parser_mem_initializer_id): Handle a C++26 pack-index-specifier. (cp_parser_simple_type_specifier): Likewise. (cp_parser_base_specifier): Likewise. * pt.cc (iterative_hash_template_arg) <case PACK_INDEX_TYPE, PACK_INDEX_EXPR>: New case. (find_parameter_packs_r) <case PACK_INDEX_TYPE, PACK_INDEX_EXPR>: New case. (make_pack_index): New. (tsubst_pack_index): New. (tsubst): Avoid tsubst on PACK_INDEX_TYPE. <case TYPENAME_TYPE>: Add a call to error. <case PACK_INDEX_TYPE>: New case. (tsubst_expr) <case PACK_INDEX_EXPR>: New case. (dependent_type_p_r): Return true for PACK_INDEX_TYPE. (type_dependent_expression_p): Recurse on PACK_INDEX_PACK for PACK_INDEX_EXPR. * ptree.cc (cxx_print_type) <case PACK_INDEX_TYPE>: New case. * semantics.cc (finish_parenthesized_expr): Set PACK_INDEX_PARENTHESIZED_P for PACK_INDEX_EXPR. (finish_type_pack_element): Adjust error messages. (pack_index_element): New. * tree.cc (cp_tree_equal) <case PACK_INDEX_EXPR>: New case. (cp_walk_subtrees) <case PACK_INDEX_TYPE, PACK_INDEX_EXPR>: New case. * typeck.cc (structural_comptypes) <case PACK_INDEX_TYPE>: New case. libstdc++-v3/ChangeLog: * testsuite/20_util/tuple/element_access/get_neg.cc: Adjust dg-prune-output. gcc/testsuite/ChangeLog: * g++.dg/cpp26/pack-indexing1.C: New test. * g++.dg/cpp26/pack-indexing2.C: New test. * g++.dg/cpp26/pack-indexing3.C: New test. * g++.dg/cpp26/pack-indexing4.C: New test. * g++.dg/cpp26/pack-indexing5.C: New test. * g++.dg/cpp26/pack-indexing6.C: New test. * g++.dg/cpp26/pack-indexing7.C: New test. * g++.dg/cpp26/pack-indexing8.C: New test. * g++.dg/cpp26/pack-indexing9.C: New test. * g++.dg/cpp26/pack-indexing10.C: New test. * g++.dg/cpp26/pack-indexing11.C: New test. * g++.dg/modules/pack-index-1_a.C: New test. * g++.dg/modules/pack-index-1_b.C: New test.
