https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101764
--- Comment #2 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:c8b2b89358481d36755dbc99e585a251780453b0 commit r12-3488-gc8b2b89358481d36755dbc99e585a251780453b0 Author: Patrick Palka <ppa...@redhat.com> Date: Mon Sep 13 10:29:32 2021 -0400 c++: parameter pack inside constexpr if [PR101764] Here when partially instantiating the first pack expansion, substitution into the condition of the constexpr if yields a still-dependent tree, so tsubst_expr returns an IF_STMT with an unsubstituted IF_COND and with IF_STMT_EXTRA_ARGS added to. Hence after partial instantiation the pack expansion pattern still refers to the unlowered parameter pack 'ts' of level 2, and it's thusly recorded in the new PACK_EXPANSION_PARAMETER_PACKS. During the subsequent final instantiation of the regenerated lambda we crash in tsubst_pack_expansion because it can't find an argument pack for this unlowered 'ts', due to the level mismatch. (Likewise when the constexpr if is replaced by a requires-expr, which also uses the extra args mechanism for avoiding partial instantiation.) So essentially, a pack expansion pattern that contains an "extra args" tree doesn't play well with partial instantiation. This patch fixes this by forcing such pack expansions to use the extra args mechanism as well. PR c++/101764 gcc/cp/ChangeLog: * cp-tree.h (PACK_EXPANSION_FORCE_EXTRA_ARGS_P): New accessor macro. * pt.c (has_extra_args_mechanism_p): New function. (find_parameter_pack_data::found_extra_args_tree_p): New data member. (find_parameter_packs_r): Set ppd->found_extra_args_tree_p appropriately. (make_pack_expansion): Set PACK_EXPANSION_FORCE_EXTRA_ARGS_P if ppd.found_extra_args_tree_p. (use_pack_expansion_extra_args_p): Return true if there were unsubstituted packs and PACK_EXPANSION_FORCE_EXTRA_ARGS_P. (tsubst_pack_expansion): Pass the pack expansion to use_pack_expansion_extra_args_p. gcc/testsuite/ChangeLog: * g++.dg/cpp1z/constexpr-if35.C: New test.
