From 76347a14801247112a34b043d9afd12d225751d5 Mon Sep 17 00:00:00 2001
From: Casey Carter <Casey@Carter.net>
Date: Sat, 4 Feb 2023 21:26:27 -0800
Subject: [PATCH 1/2] Use `if constexpr` dispatch for `_Pass_fn`

---
 stl/inc/xutility | 18 ++++++++----------
 1 file changed, 8 insertions(+), 10 deletions(-)

diff --git a/stl/inc/xutility b/stl/inc/xutility
index 235fd627b38..95cdb60ba44 100644
--- a/stl/inc/xutility
+++ b/stl/inc/xutility
@@ -360,7 +360,6 @@ _EXPORT_STD struct identity {
 };
 #endif // _HAS_CXX20
 
-// TRANSITION, VSO-386225
 template <class _Fx>
 struct _Ref_fn { // pass function object by value as a reference
     template <class... _Args>
@@ -376,17 +375,16 @@ struct _Ref_fn { // pass function object by value as a reference
 };
 
 template <class _Fn>
-_INLINE_VAR constexpr bool _Pass_functor_by_value_v = conjunction_v<bool_constant<sizeof(_Fn) <= sizeof(void*)>,
+_INLINE_VAR constexpr bool _Pass_functor_by_value = conjunction_v<bool_constant<sizeof(_Fn) <= sizeof(void*)>,
     is_trivially_copy_constructible<_Fn>, is_trivially_destructible<_Fn>>;
 
-template <class _Fn, enable_if_t<_Pass_functor_by_value_v<_Fn>, int> = 0> // TRANSITION, if constexpr
-constexpr _Fn _Pass_fn(_Fn _Val) { // pass functor by value
-    return _Val;
-}
-
-template <class _Fn, enable_if_t<!_Pass_functor_by_value_v<_Fn>, int> = 0>
-constexpr _Ref_fn<_Fn> _Pass_fn(_Fn& _Val) { // pass functor by "reference"
-    return {_Val};
+template <class _Fn>
+constexpr auto _Pass_fn(_Fn& _Func) {
+    if constexpr (_Pass_functor_by_value<_Fn>) {
+        return _Func; // pass functor by value
+    } else {
+        return _Ref_fn<_Fn>{_Func}; // pass functor by "reference"
+    }
 }
 
 #if _HAS_CXX23

From 28b56980f79a7dae974b5bf0a26dc94f695541c0 Mon Sep 17 00:00:00 2001
From: Casey Carter <Casey@Carter.net>
Date: Sun, 5 Feb 2023 17:11:42 -0800
Subject: [PATCH 2/2] miscco's review comments

---
 stl/inc/xutility | 12 +++++-------
 1 file changed, 5 insertions(+), 7 deletions(-)

diff --git a/stl/inc/xutility b/stl/inc/xutility
index 95cdb60ba44..9e613dc1b70 100644
--- a/stl/inc/xutility
+++ b/stl/inc/xutility
@@ -375,13 +375,11 @@ struct _Ref_fn { // pass function object by value as a reference
 };
 
 template <class _Fn>
-_INLINE_VAR constexpr bool _Pass_functor_by_value = conjunction_v<bool_constant<sizeof(_Fn) <= sizeof(void*)>,
-    is_trivially_copy_constructible<_Fn>, is_trivially_destructible<_Fn>>;
-
-template <class _Fn>
-constexpr auto _Pass_fn(_Fn& _Func) {
-    if constexpr (_Pass_functor_by_value<_Fn>) {
-        return _Func; // pass functor by value
+constexpr auto _Pass_fn(_Fn& _Func) noexcept {
+    constexpr bool _Pass_by_value = conjunction_v<bool_constant<sizeof(_Fn) <= sizeof(void*)>,
+        is_trivially_copy_constructible<_Fn>, is_trivially_destructible<_Fn>>;
+    if constexpr (_Pass_by_value) {
+        return _Func;
     } else {
         return _Ref_fn<_Fn>{_Func}; // pass functor by "reference"
     }