On Tue, 24 Oct 2023 at 03:16, Ken Matsui <kmatsui@gcc.gnu.org> wrote:

> This patch optimizes the compilation performance of std::is_function
> by dispatching to the new __is_function built-in trait.
>
> libstdc++-v3/ChangeLog:
>
>         * include/std/type_traits (is_function): Use __is_function
>         built-in trait.
>         (is_function_v): Likewise. Optimize its implementation.
>         (is_const_v): Move on top of is_function_v as is_function_v now
>         depends on is_const_v.
>

I think I'd prefer to keep is_const_v where it is now, adjacent to
is_volatile_v, and move is_function_v after those.

i.e. like this (but with the additional changes to use the new built-in):

--- a/libstdc++-v3/include/std/type_traits
+++ b/libstdc++-v3/include/std/type_traits
@@ -3198,8 +3198,8 @@ template <typename _Tp>
   inline constexpr bool is_union_v = __is_union(_Tp);
 template <typename _Tp>
   inline constexpr bool is_class_v = __is_class(_Tp);
-template <typename _Tp>
-  inline constexpr bool is_function_v = is_function<_Tp>::value;
+// is_function_v is defined below, after is_const_v.
+
 template <typename _Tp>
   inline constexpr bool is_reference_v = false;
 template <typename _Tp>
@@ -3226,6 +3226,8 @@ template <typename _Tp>
   inline constexpr bool is_volatile_v = false;
 template <typename _Tp>
   inline constexpr bool is_volatile_v<volatile _Tp> = true;
+template <typename _Tp>
+  inline constexpr bool is_function_v = is_function<_Tp>::value;

 template <typename _Tp>
   inline constexpr bool is_trivial_v = __is_trivial(_Tp);

The variable templates are currently defined in the order shown in the
standard, in te [meta.type.synop] synopsis, and in the [meta.unary.cat]
table. So let's move _is_function_v later and add a comment saying why it's
not in the expected place.