https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100138
--- 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:2e63d08cb9bc001232734eed32e4bc3814a279a9 commit r11-8761-g2e63d08cb9bc001232734eed32e4bc3814a279a9 Author: Patrick Palka <ppa...@redhat.com> Date: Mon May 10 22:33:04 2021 -0400 c++: fn parm pack expansion inside constraint [PR100138] This PR is about CTAD but the underlying problems are more general; CTAD is a good trigger for them because of the necessary substitution into constraints that deduction guide generation entails. In the testcase below, when generating the implicit deduction guide for the constrained constructor template for A, we substitute the generic flattening map 'tsubst_args' into the constructor's constraints. During this substitution, tsubst_pack_expansion returns a rebuilt pack expansion for sizeof...(xs), but doesn't carry over the PACK_EXPANSION_LOCAL_P (and PACK_EXPANSION_SIZEOF_P) flag from the original tree to the rebuilt one. The flag is otherwise unset on the original tree but gets set for the rebuilt tree from make_pack_expansion since at_function_scope_p() is true (we're inside main). This leads to a crash during satisfaction when substituting into the pack expansion because we don't have local_specializations set up (and it'd be set up for us if PACK_EXPANSION_LOCAL_P is unset) Similarly, tsubst_constraint needs to set cp_unevaluated so that the substitution performed therein doesn't rely on local_specializations. This avoids a crash during CTAD for C below. gcc/cp/ChangeLog: PR c++/100138 * constraint.cc (tsubst_constraint): Set up cp_unevaluated. (satisfy_atom): Set up iloc_sentinel before calling cxx_constant_value. * pt.c (tsubst_pack_expansion): When returning a rebuilt pack expansion, carry over PACK_EXPANSION_LOCAL_P and PACK_EXPANSION_SIZEOF_P from the original pack expansion. gcc/testsuite/ChangeLog: PR c++/100138 * g++.dg/cpp2a/concepts-ctad4.C: New test. (cherry picked from commit e7a9f085ffd34b0d7bc4b803c182b41494f609aa)
