https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110349
--- Comment #8 from GCC Commits <cvs-commit at gcc dot gnu.org> --- The master branch has been updated by Jakub Jelinek <ja...@gcc.gnu.org>: https://gcc.gnu.org/g:6c9973e46bdb4496b5af8a7170269e91e36ad35a commit r14-5991-g6c9973e46bdb4496b5af8a7170269e91e36ad35a Author: Jakub Jelinek <ja...@redhat.com> Date: Thu Nov 30 09:09:21 2023 +0100 c++: Implement C++26 P2169R4 - Placeholder variables with no name [PR110349] The following patch implements the C++26 P2169R4 paper. As written in the PR, the patch expects that: 1) https://eel.is/c++draft/expr.prim.lambda.capture#2 "Ignoring appearances in initializers of init-captures, an identifier or this shall not appear more than once in a lambda-capture." is adjusted such that name-independent lambda captures with initializers can violate this rule (but lambda captures which aren't name-independent can't appear after name-independent ones) 2) https://eel.is/c++draft/class.mem#general-5 "A member shall not be declared twice in the member-specification, except that" having an exception that name-independent non-static data member declarations can appear multiple times (but again, if there is a member which isn't name-independent, it can't appear after name-independent ones) 3) it assumes that any name-independent declarations which weren't previously valid result in the _ lookups being ambiguous, not just if there are 2 _ declarations in the same scope, in particular the https://eel.is/c++draft/basic.scope#block-2 mentioned cases 4) it assumes that _ in static function/block scope structured bindings is never name-independent like in namespace scope structured bindings; it matches clang behavior and is consistent with e.g. static type _; not being name-independent both at namespace scope and at function/block scope As you preferred in the PR, for local scope bindings, the ambiguous cases use a TREE_LIST with the ambiguous cases which can often be directly fed into print_candidates. For member_vec after sorting/deduping, I chose to use instead OVERLOAD with a new flag but only internally inside of the member_vec, get_class_binding_direct turns it into a TREE_LIST. There are 2 reasons for that, in order to keep the member_vec binary search fast, I think it is better to keep OVL_NAME usable on all elements because having to special case TREE_LIST would slow everything down, and the callers need to be able to chain the results anyway and so need an unshared TREE_LIST they can tweak/destroy anyway. name-independent declarations (even in older standards) will not have -Wunused{,-variable,-but-set-variable} or -Wshadow* warnings diagnosed, but unlike e.g. the clang implementation, this patch does diagnose -Wunused-parameter for parameters with _ names because they aren't name-independent and one can just omit their name instead. 2023-11-30 Jakub Jelinek <ja...@redhat.com> PR c++/110349 gcc/c-family/ * c-cppbuiltin.cc (c_cpp_builtins): Predefine __cpp_placeholder_variables=202306L for C++26. gcc/cp/ * cp-tree.h: Implement C++26 P2169R4 - Placeholder variables with no name. (OVL_NAME_INDEPENDENT_DECL_P): Define. (add_capture): Add unsigned * argument. (name_independent_decl_p): New inline function. * name-lookup.cc (class name_lookup): Make ambiguous and add_value members public. (name_independent_linear_search): New function. (get_class_binding_direct): Handle member_vec_binary_search returning OVL_NAME_INDEPENDENT_DECL_P OVERLOAD. Use name_independent_linear_search rather than fields_linear_search for linear lookup of _ name if !want_type. (member_name_cmp): Sort name-independent declarations first. (member_vec_dedup): Handle name-independent declarations. (pop_local_binding): Handle binding->value being a TREE_LIST for ambiguous name-independent declarations. (supplement_binding): Handle name-independent declarations. (update_binding): Likewise. (check_local_shadow): Return tree rather than void, normally NULL_TREE but old for name-independent declarations which used to conflict with outer scope declaration. Don't emit -Wshadow* warnings for name-independent declarations. (pushdecl): Handle name-independent declarations. * search.cc (lookup_field_r): Handle nval being a TREE_LIST. * lambda.cc (build_capture_proxy): Adjust for ___.<number> names of members. (add_capture): Add NAME_INDEPENDENT_CNT argument. Use ___.<number> name rather than ___ for second and following capture with _ name. (add_default_capture): Adjust add_capture caller. * decl.cc (poplevel): Don't warn about name-independent declarations. (duplicate_decls): If in C++26 a _ named declaration conflicts with earlier declarations, emit explaining note why the new declaration is not name-independent. (reshape_init_class): If field is a TREE_LIST, emit an ambiguity error with list of candidates rather than error about non-existing non-static data member. * parser.cc (cp_parser_lambda_introducer): Adjust add_capture callers. Allow name-independent capture redeclarations. (cp_parser_decomposition_declaration): Set decl_specs.storage_class to sc_static for static structured bindings. * pt.cc (tsubst_lambda_expr): Adjust add_capture caller. gcc/testsuite/ * g++.dg/cpp26/name-independent-decl1.C: New test. * g++.dg/cpp26/name-independent-decl2.C: New test. * g++.dg/cpp26/name-independent-decl3.C: New test. * g++.dg/cpp26/name-independent-decl4.C: New test. * g++.dg/cpp26/name-independent-decl5.C: New test. * g++.dg/cpp26/name-independent-decl6.C: New test. * g++.dg/cpp26/feat-cxx26.C: Add __cpp_placeholder_variables test.
