https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110323
--- Comment #10 from GCC Commits <cvs-commit at gcc dot gnu.org> --- The trunk branch has been updated by Marek Polacek <mpola...@gcc.gnu.org>: https://gcc.gnu.org/g:081f8937cb82da311c224da04b0c6cbd57a8fb5d commit r14-9596-g081f8937cb82da311c224da04b0c6cbd57a8fb5d Author: Marek Polacek <pola...@redhat.com> Date: Thu Mar 7 20:41:23 2024 -0500 c++: explicit inst of template method not generated [PR110323] Consider constexpr int VAL = 1; struct foo { template <int B> void bar(typename std::conditional<B==VAL, int, float>::type arg) { } }; template void foo::bar<1>(int arg); where we since r11-291 fail to emit the code for the explicit instantiation. That's because cp_walk_subtrees/TYPENAME_TYPE now walks TYPE_CONTEXT ('conditional' here) as well, and in a template finds the B==VAL template argument. VAL is constexpr, which implies const, which in the global scope implies static. constrain_visibility_for_template then makes "struct conditional<(B == VAL), int, float>" non-TREE_PUBLIC. Then symtab_node::needed_p checks TREE_PUBLIC, sees it's 0, and we don't emit any code. I thought the fix would be some ODR-esque check to not consider constexpr variables/fns that are used just for their value. But it turned out to be tricky. For instance, we can't skip determine_visibility in a template; we can't even skip it for value-dep expressions. For example, no-linkage-expr1.C has using P = struct {}*; template <int N> void f(int(*)[((P)0, N)]) {} where ((P)0, N) is value-dep, but N is not relevant here: we have to ferret out the anonymous type. When instantiating, it's already gone. This patch uses decl_constant_var_p. This is to implement (an approximation) [basic.def.odr]#14.5.1 and [basic.def.odr]#5.2. PR c++/110323 gcc/cp/ChangeLog: * decl2.cc (min_vis_expr_r) <case VAR_DECL>: Do nothing for decl_constant_var_p VAR_DECLs. gcc/testsuite/ChangeLog: * g++.dg/template/explicit-instantiation6.C: New test. * g++.dg/template/explicit-instantiation7.C: New test.
