Re: [PATCH] c++: explicit inst of template method not generated [PR110323]

[Jason Merrill <jason@redhat.com>]

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.

Jason