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

[Marek Polacek <polacek@redhat.com>]

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