https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105268
--- Comment #3 from CVS 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:ccae4443c8322d9d82a19a1e7c689413a122a478 commit r12-8176-gccae4443c8322d9d82a19a1e7c689413a122a478 Author: Marek Polacek <pola...@redhat.com> Date: Thu Apr 14 13:48:15 2022 -0400 c++: wrong error with variadic concept [PR105268] Here we issue a wrong error for the template<typename T = S<C_many<int>>> void g(); line in the testcase. I surmise that's because we mistakenly parse C_many<int> as a placeholder-type-specifier, and things go wrong from there. We are in a default argument so we should reject parsing C_many<int> as a placeholder-type-specifier, which would mean creating a new parameter. We want C_many<int> to be a concept-id instead. It's interesting to see why the same problem didn't occur for C_one<int>. In that case, cp_parser_placeholder_type_specifier -> finish_type_constraints -> build_type_constraint -> build_concept_check -> build_standard_check -> coerce_template_parms fails the parse here: 8916 nargs = inner_args ? NUM_TMPL_ARGS (inner_args) : 0; 8917 if ((nargs - variadic_args_p > nparms && !variadic_p) 8918 || (nargs < nparms - variadic_p 8919 && require_all_args 8920 && !variadic_args_p 8921 && (!use_default_args 8922 || (TREE_VEC_ELT (parms, nargs) != error_mark_node 8923 && !TREE_PURPOSE (TREE_VEC_ELT (parms, nargs)))))) 8924 { 8925 bad_nargs: ... 8943 return error_mark_node; because nargs is 2 (the targs are <WILDCARD_DECL, int>) while nparms is 1 (for the one 'typename' in the tparam list of C_one). But for C_many<int> variadic_p is true so we don't return error_mark_node but <type_argument_pack>. This patch does not issue any error for the !tentative case because I didn't figure out how to trigger that. So it adds an assert instead. PR c++/105268 gcc/cp/ChangeLog: * parser.cc (cp_parser_placeholder_type_specifier): Return error_mark_node when trying to build up a constrained parameter in a default argument. gcc/testsuite/ChangeLog: * g++.dg/concepts/variadic6.C: New test.
