https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112737
--- Comment #8 from GCC 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:3ba5be16a2be3eaedf2870ca1e25cfe826945948 commit r14-8705-g3ba5be16a2be3eaedf2870ca1e25cfe826945948 Author: Patrick Palka <ppa...@redhat.com> Date: Thu Feb 1 13:17:48 2024 -0500 c++: ttp TEMPLATE_DECL equivalence [PR112737] Here during declaration matching we undesirably consider the two TT{42} CTAD expressions to be non-equivalent ultimately because for CTAD placeholder equivalence we compare the TEMPLATE_DECLs via pointer identity, and here the corresponding TEMPLATE_DECLs for TT are different since they're from different scopes. On the other hand, the corresponding TEMPLATE_TEMPLATE_PARMs are deemed equivalent according to cp_tree_equal (since they have the same position and template parameters). This turns out to be the root cause of some of the xtreme-header modules regressions. So this patch relaxes ttp CTAD placeholder equivalence accordingly, by comparing the TEMPLATE_TEMPLATE_PARM instead of the TEMPLATE_DECL. It turns out this issue also affects function template-id equivalence as with g<TT> in the second testcase, so it makes sense to relax TEMPLATE_DECL equivalence more generally in cp_tree_equal. In passing this patch improves ctp_hasher::hash for CTAD placeholders, so that they don't all get the same hash. PR c++/112737 gcc/cp/ChangeLog: * pt.cc (iterative_hash_template_arg) <case TEMPLATE_DECL>: Adjust hashing to match cp_tree_equal. (ctp_hasher::hash): Also hash CLASS_PLACEHOLDER_TEMPLATE. * tree.cc (cp_tree_equal) <case TEMPLATE_DECL>: Return true for ttp TEMPLATE_DECLs if their TEMPLATE_TEMPLATE_PARMs are equivalent. * typeck.cc (structural_comptypes) <case TEMPLATE_TYPE_PARM>: Use cp_tree_equal to compare CLASS_PLACEHOLDER_TEMPLATE. gcc/testsuite/ChangeLog: * g++.dg/template/ttp42.C: New test. * g++.dg/template/ttp43.C: New test. Reviewed-by: Jason Merrill <ja...@redhat.com>
