[Bug c++/110323] [11/12/13/14 Regression] Code for explicit instantiation of template method of template class not generated

["mpolacek at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110323

--- Comment #8 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
(In reply to Patrick Palka from comment #7)
> I noticed we emit the function if we turn it into a non-member:
> 
> #include <type_traits>
> 
> constexpr int VAL = 1;
> 
>     template <int B>
>     void bar(typename std::conditional<B==VAL, int, float>::type arg) {
>     }
> 
> template void bar<1>(int arg);
> 
> I wonder why this bug seems specific to member functions?

Good question.  It's because in this case bar is marked as force_output here:

  /* When not optimizing, also output the static functions. (see
     PR24561), but don't do so for always_inline functions, functions
     declared inline and nested functions.  These were optimized out
     in the original implementation and it is unclear whether we want
     to change the behavior here.  */
  if (((!opt_for_fn (decl, optimize) || flag_keep_static_functions
        || node->no_reorder)
       && !node->cpp_implicit_alias
       && !DECL_DISREGARD_INLINE_LIMITS (decl)
       && !DECL_DECLARED_INLINE_P (decl)
       && !(DECL_CONTEXT (decl)
            && TREE_CODE (DECL_CONTEXT (decl)) == FUNCTION_DECL))
      && !DECL_COMDAT (decl) && !DECL_EXTERNAL (decl))
    node->force_output = 1;

and when deciding if we ought to emit the fn in symtab_node::needed_p we do:

  /* If the user told us it is used, then it must be so.  */
  if (force_output)
    return true;


With -O the fn isn't emitted.