On Thu, Mar 14, 2024 at 03:39:04PM -0400, Jason Merrill wrote: > On 3/8/24 12:02, Marek Polacek wrote: > > Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk? > > > > -- >8 -- > > 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. > > Hmm, how is that different from the B == VAL case? In both cases we're > naming an internal entity that gets folded away. > > I guess the difference is that B == VAL falls under the special allowance in > https://eel.is/c++draft/basic.def.odr#14.5.1 because it's a constant used as > a prvalue, and therefore is not odr-used under > https://eel.is/c++draft/basic.def.odr#5.2 > > So I would limit this change to decl_constant_var_p. Really we should also > be checking that the lvalue-rvalue conversion is applied, but that's more > complicated.
Thanks. My previous version had it, but it didn't handle static constexpr int getval () { return 1; } template <int B> void baz(typename conditional<B == getval (), int, float>::type arg) { } I'd say that "getval()" is one of "manifestly constant-evaluated expressions that are not value-dependent", so it should be treated the same as B == VAL. I don't know if this is important to handle. Do you want me to poke further or should we just go with decl_constant_var_p and leave it at that for now? Marek