[llvm] [STLExtras] Backport std::to_underlying from C++23 (PR #70681)

[Vlad Serebrennikov via llvm-commits]

https://github.com/Endilll updated https://github.com/llvm/llvm-project/pull/70681

>From a83117aaf066cb89237d51dec4d9fb23e356fd9c Mon Sep 17 00:00:00 2001
From: Vlad Serebrennikov <serebrennikov.vladislav at gmail.com>
Date: Mon, 30 Oct 2023 19:11:52 +0300
Subject: [PATCH 1/2] [STLExtras] Backport std::to_underlying from C++23

---
 llvm/include/llvm/ADT/STLExtras.h    |  6 ++++++
 llvm/unittests/ADT/STLExtrasTest.cpp | 22 ++++++++++++++++++++++
 2 files changed, 28 insertions(+)

diff --git a/llvm/include/llvm/ADT/STLExtras.h b/llvm/include/llvm/ADT/STLExtras.h
index 18bc4d108b156bf..98342bfc36e9f83 100644
--- a/llvm/include/llvm/ADT/STLExtras.h
+++ b/llvm/include/llvm/ADT/STLExtras.h
@@ -1535,6 +1535,12 @@ constexpr decltype(auto) makeVisitor(CallableTs &&...Callables) {
   return detail::Visitor<CallableTs...>(std::forward<CallableTs>(Callables)...);
 }
 
+/// Backport of C++23 std::to_underlying.
+template <typename Enum>
+[[nodiscard]] constexpr typename std::underlying_type_t<Enum> to_underlying(Enum E) {
+  return static_cast<typename std::underlying_type_t<Enum>>(E);
+}
+
 //===----------------------------------------------------------------------===//
 //     Extra additions to <algorithm>
 //===----------------------------------------------------------------------===//
diff --git a/llvm/unittests/ADT/STLExtrasTest.cpp b/llvm/unittests/ADT/STLExtrasTest.cpp
index 7db339e4ef31cdc..213b3e4b3d06fb6 100644
--- a/llvm/unittests/ADT/STLExtrasTest.cpp
+++ b/llvm/unittests/ADT/STLExtrasTest.cpp
@@ -1332,4 +1332,26 @@ struct Bar {};
 static_assert(is_incomplete_v<Foo>, "Foo is incomplete");
 static_assert(!is_incomplete_v<Bar>, "Bar is defined");
 
+TEST(STLExtrasTest, ToUnderlying) {
+  enum E {
+    A1 = 0, B1 = -1
+  };
+  static_assert(to_underlying(A1) == 0);
+  static_assert(to_underlying(B1) == -1);
+
+  enum E2 : unsigned char {
+    A2 = 0, B2
+  };
+  static_assert(std::is_same_v<unsigned char, decltype(to_underlying(A2))>);
+  static_assert(to_underlying(A2) == 0);
+  static_assert(to_underlying(B2) == 1);
+
+  enum class E3 {
+    A3 = -1, B3
+  };
+  static_assert(std::is_same_v<int, decltype(to_underlying(E3::A3))>);
+  static_assert(to_underlying(E3::A3) == -1);
+  static_assert(to_underlying(E3::B3) == 0);
+}
+
 } // namespace

>From 1fdc7025a213d7418cdeff984a4162de415be49e Mon Sep 17 00:00:00 2001
From: Vlad Serebrennikov <serebrennikov.vladislav at gmail.com>
Date: Mon, 30 Oct 2023 19:17:21 +0300
Subject: [PATCH 2/2] Run clang-format

---
 llvm/include/llvm/ADT/STLExtras.h    |  3 ++-
 llvm/unittests/ADT/STLExtrasTest.cpp | 12 +++---------
 2 files changed, 5 insertions(+), 10 deletions(-)

diff --git a/llvm/include/llvm/ADT/STLExtras.h b/llvm/include/llvm/ADT/STLExtras.h
index 98342bfc36e9f83..513837582072fa0 100644
--- a/llvm/include/llvm/ADT/STLExtras.h
+++ b/llvm/include/llvm/ADT/STLExtras.h
@@ -1537,7 +1537,8 @@ constexpr decltype(auto) makeVisitor(CallableTs &&...Callables) {
 
 /// Backport of C++23 std::to_underlying.
 template <typename Enum>
-[[nodiscard]] constexpr typename std::underlying_type_t<Enum> to_underlying(Enum E) {
+[[nodiscard]] constexpr typename std::underlying_type_t<Enum>
+to_underlying(Enum E) {
   return static_cast<typename std::underlying_type_t<Enum>>(E);
 }
 
diff --git a/llvm/unittests/ADT/STLExtrasTest.cpp b/llvm/unittests/ADT/STLExtrasTest.cpp
index 213b3e4b3d06fb6..7bf7a7d22670127 100644
--- a/llvm/unittests/ADT/STLExtrasTest.cpp
+++ b/llvm/unittests/ADT/STLExtrasTest.cpp
@@ -1333,22 +1333,16 @@ static_assert(is_incomplete_v<Foo>, "Foo is incomplete");
 static_assert(!is_incomplete_v<Bar>, "Bar is defined");
 
 TEST(STLExtrasTest, ToUnderlying) {
-  enum E {
-    A1 = 0, B1 = -1
-  };
+  enum E { A1 = 0, B1 = -1 };
   static_assert(to_underlying(A1) == 0);
   static_assert(to_underlying(B1) == -1);
 
-  enum E2 : unsigned char {
-    A2 = 0, B2
-  };
+  enum E2 : unsigned char { A2 = 0, B2 };
   static_assert(std::is_same_v<unsigned char, decltype(to_underlying(A2))>);
   static_assert(to_underlying(A2) == 0);
   static_assert(to_underlying(B2) == 1);
 
-  enum class E3 {
-    A3 = -1, B3
-  };
+  enum class E3 { A3 = -1, B3 };
   static_assert(std::is_same_v<int, decltype(to_underlying(E3::A3))>);
   static_assert(to_underlying(E3::A3) == -1);
   static_assert(to_underlying(E3::B3) == 0);