https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105268
--- Comment #3 from CVS Commits ---
The trunk branch has been updated by Marek Polacek :
https://gcc.gnu.org/g:ccae4443c8322d9d82a19a1e7c689413a122a478
commit r12-8176-gccae4443c8322d9d82a19a1e7c689413a122a478
Author: Marek Polacek
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>> void g();
line in the testcase. I surmise that's because we mistakenly parse
C_many as a placeholder-type-specifier, and things go wrong from
there. We are in a default argument so we should reject parsing
C_many
as a placeholder-type-specifier, which would mean creating a new parameter.
We want C_many to be a concept-id instead.
It's interesting to see why the same problem didn't occur for C_one.
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 ) while nparms is
1 (for the one 'typename' in the tparam list of C_one). But for
C_many variadic_p is true so we don't return error_mark_node but
.
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.
