https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99904
--- Comment #3 from CVS Commits <cvs-commit at gcc dot gnu.org> --- The releases/gcc-11 branch has been updated by Patrick Palka <ppa...@gcc.gnu.org>: https://gcc.gnu.org/g:052b2df6f0ecd1e5b55056911b16bca8b31ef7df commit r11-9088-g052b2df6f0ecd1e5b55056911b16bca8b31ef7df Author: Patrick Palka <ppa...@redhat.com> Date: Thu Oct 7 10:02:54 2021 -0400 c++: variadic ttp constraint subsumption [PR99904] Here we're crashing when level-lowering the variadic constraint C<Ts...> on the template template parameter TT because tsubst_pack_expansion expects processing_template_decl to be set during a partial substitution. PR c++/99904 gcc/cp/ChangeLog: * pt.c (is_compatible_template_arg): Set processing_template_decl around tsubst_constraint_info. gcc/testsuite/ChangeLog: * g++.dg/cpp2a/concepts-ttp4.C: New test. (cherry picked from commit 2e6e0d86a06389056d0e7fecc99c547420ad787a)