From afe8a925611ff16bbfc8a97219771e6bdc1192d7 Mon Sep 17 00:00:00 2001
From: Daniel Marshall <xandan@gmail.com>
Date: Sat, 9 Nov 2019 17:51:43 +0000
Subject: [PATCH 1/4] Make underlying_type SFINAE-friendly

---
 stl/inc/type_traits  | 11 ++++++++++-
 stl/inc/yvals_core.h |  1 +
 2 files changed, 11 insertions(+), 1 deletion(-)

diff --git a/stl/inc/type_traits b/stl/inc/type_traits
index 7adf7b62d57..44fbf7d3798 100644
--- a/stl/inc/type_traits
+++ b/stl/inc/type_traits
@@ -1151,11 +1151,20 @@ template <size_t _Len, class... _Types>
 using aligned_union_t = typename aligned_union<_Len, _Types...>::type;
 
 // STRUCT TEMPLATE underlying_type
+template <class _Ty, bool = is_enum_v<_Ty>>
+struct _Underlying_type {};
+
+template <class _Ty>
+struct _Underlying_type<_Ty, false> {};
+
 template <class _Ty>
-struct underlying_type { // determine underlying type for enum
+struct _Underlying_type<_Ty, true> {
     using type = __underlying_type(_Ty);
 };
 
+template <class _Ty>
+struct underlying_type : _Underlying_type<_Ty> {}; // determine underlying type for enum
+
 template <class _Ty>
 using underlying_type_t = typename underlying_type<_Ty>::type;
 
diff --git a/stl/inc/yvals_core.h b/stl/inc/yvals_core.h
index 1da8c63ed42..3ebe1794020 100644
--- a/stl/inc/yvals_core.h
+++ b/stl/inc/yvals_core.h
@@ -149,6 +149,7 @@
 // P0063R3 C11 Standard Library
 // P0074R0 owner_less<>
 // P0092R1 <chrono> floor(), ceil(), round(), abs()
+// P0340R3 SFINAE-Friendly underlying_type
 // P0414R2 shared_ptr<T[]>, shared_ptr<T[N]>
 // P0418R2 atomic compare_exchange memory_order Requirements
 // P0435R1 Overhauling common_type

From e26e8366c6aa62c4ff74b3379d7404808be1d8c8 Mon Sep 17 00:00:00 2001
From: Daniel Marshall <xandan@gmail.com>
Date: Sat, 9 Nov 2019 18:48:12 +0000
Subject: [PATCH 2/4] Remove unneeded specialisation

---
 stl/inc/type_traits | 3 ---
 1 file changed, 3 deletions(-)

diff --git a/stl/inc/type_traits b/stl/inc/type_traits
index 44fbf7d3798..4a7418378b8 100644
--- a/stl/inc/type_traits
+++ b/stl/inc/type_traits
@@ -1154,9 +1154,6 @@ using aligned_union_t = typename aligned_union<_Len, _Types...>::type;
 template <class _Ty, bool = is_enum_v<_Ty>>
 struct _Underlying_type {};
 
-template <class _Ty>
-struct _Underlying_type<_Ty, false> {};
-
 template <class _Ty>
 struct _Underlying_type<_Ty, true> {
     using type = __underlying_type(_Ty);

From 56bfa4f1efa37f3c58293c72af129d423a743ba2 Mon Sep 17 00:00:00 2001
From: "Stephan T. Lavavej" <stl@microsoft.com>
Date: Mon, 11 Nov 2019 17:09:01 -0800
Subject: [PATCH 3/4] Use the primary template for the true case.

This improves readability.
---
 stl/inc/type_traits | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/stl/inc/type_traits b/stl/inc/type_traits
index 4a7418378b8..1796b8a46e2 100644
--- a/stl/inc/type_traits
+++ b/stl/inc/type_traits
@@ -1152,13 +1152,13 @@ using aligned_union_t = typename aligned_union<_Len, _Types...>::type;
 
 // STRUCT TEMPLATE underlying_type
 template <class _Ty, bool = is_enum_v<_Ty>>
-struct _Underlying_type {};
-
-template <class _Ty>
-struct _Underlying_type<_Ty, true> {
+struct _Underlying_type {
     using type = __underlying_type(_Ty);
 };
 
+template <class _Ty>
+struct _Underlying_type<_Ty, false> {};
+
 template <class _Ty>
 struct underlying_type : _Underlying_type<_Ty> {}; // determine underlying type for enum
 

From 2efa287f0ac8effdfe7c9436b2fef0b253d548f4 Mon Sep 17 00:00:00 2001
From: "Stephan T. Lavavej" <stl@microsoft.com>
Date: Mon, 11 Nov 2019 19:39:02 -0800
Subject: [PATCH 4/4] Avoid an instantiation, micro-optimizing throughput.

---
 stl/inc/type_traits | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/stl/inc/type_traits b/stl/inc/type_traits
index 1796b8a46e2..190994f281a 100644
--- a/stl/inc/type_traits
+++ b/stl/inc/type_traits
@@ -1163,7 +1163,7 @@ template <class _Ty>
 struct underlying_type : _Underlying_type<_Ty> {}; // determine underlying type for enum
 
 template <class _Ty>
-using underlying_type_t = typename underlying_type<_Ty>::type;
+using underlying_type_t = typename _Underlying_type<_Ty>::type;
 
 // STRUCT TEMPLATE rank
 template <class _Ty>