On 4/14/22 16:24, Marek Polacek wrote:
Here we issue a wrong error for thetemplate<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.
Then I'd add an assert that tentative is true. OK with that change.
Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk? 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. --- gcc/cp/parser.cc | 7 ++++++- gcc/testsuite/g++.dg/concepts/variadic6.C | 20 ++++++++++++++++++++ 2 files changed, 26 insertions(+), 1 deletion(-) create mode 100644 gcc/testsuite/g++.dg/concepts/variadic6.C diff --git a/gcc/cp/parser.cc b/gcc/cp/parser.cc index bfd16e1ef62..dfb613168b6 100644 --- a/gcc/cp/parser.cc +++ b/gcc/cp/parser.cc @@ -20041,7 +20041,12 @@ cp_parser_placeholder_type_specifier (cp_parser *parser, location_t loc, /* In a template parameter list, a type-parameter can be introduced by type-constraints alone. */ if (processing_template_parmlist && !placeholder) - return build_constrained_parameter (con, proto, args); + { + /* In a default argument we may not be creating new parameters. */ + if (parser->local_variables_forbidden_p & LOCAL_VARS_FORBIDDEN) + return error_mark_node; + return build_constrained_parameter (con, proto, args); + }/* Diagnose issues placeholder issues. */if (!flag_concepts_ts diff --git a/gcc/testsuite/g++.dg/concepts/variadic6.C b/gcc/testsuite/g++.dg/concepts/variadic6.C new file mode 100644 index 00000000000..0b386b0cd6d --- /dev/null +++ b/gcc/testsuite/g++.dg/concepts/variadic6.C @@ -0,0 +1,20 @@ +// PR c++/105268 +// { dg-do compile { target concepts } } + +template<typename> concept C_one = true; +template<typename...> concept C_many = true; + +template<bool B> struct S { }; + +template<typename T = S<C_one<int>>> void f(); +template<typename T = S<C_many<int>>> void g(); + +void +fn (auto a = S<C_one<int>>{}) +{ +} + +void +fn2 (auto a = S<C_many<int>>{}) +{ +} base-commit: 82536fbb8a7d150b829650378e0ba07dad5c8fb8
