https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107853
--- Comment #5 from CVS Commits <cvs-commit at gcc dot gnu.org> --- The master branch has been updated by Patrick Palka <ppa...@gcc.gnu.org>: https://gcc.gnu.org/g:bd1fc4a219d8c0fad0ec41002e895b49e384c1c2 commit r13-4876-gbd1fc4a219d8c0fad0ec41002e895b49e384c1c2 Author: Patrick Palka <ppa...@redhat.com> Date: Fri Dec 23 09:18:37 2022 -0500 c++: template friend with variadic constraints [PR107853] When instantiating a constrained hidden template friend, we substitute into its template-head requirements in tsubst_friend_function. For this substitution we use the template's full argument vector whose outer levels correspond to the instantiated class's arguments and innermost level corresponds to the template's own level-lowered generic arguments. But for A<int>::f here, for which the relevant argument vector is {{int}, {Us...}}, the substitution into (C<Ts, Us> && ...) triggers the assert in use_pack_expansion_extra_args_p since one argument is a pack expansion and the other isn't. And for A<int, int>::f, for which the relevant argument vector is {{int, int}, {Us...}}, the use_pack_expansion_extra_args_p assert would also trigger but we first get a bogus "mismatched argument pack lengths" error from tsubst_pack_expansion. Sidestepping the question of whether tsubst_pack_expansion should be able to handle such substitutions, it seems we can work around this by using only the instantiated class's arguments and not also the template friend's own generic arguments, which is consistent with how we normally substitute into the signature of a member template. PR c++/107853 gcc/cp/ChangeLog: * constraint.cc (maybe_substitute_reqs_for): Substitute into the template-head requirements of a template friend using only its outer arguments via outer_template_args. * cp-tree.h (outer_template_args): Declare. * pt.cc (outer_template_args): Define, factored out and generalized from ... (ctor_deduction_guides_for): ... here. gcc/testsuite/ChangeLog: * g++.dg/cpp2a/concepts-friend12.C: New test. * g++.dg/cpp2a/concepts-friend13.C: New test.
