From fc5b252010aeb6155d22e752914b8b9e371c5398 Mon Sep 17 00:00:00 2001
From: Brian Gianforcaro <b.gianfo@gmail.com>
Date: Thu, 4 Mar 2021 23:16:59 -0800
Subject: [PATCH] AK: Implement IsEnum<T> and UnderlyingType<T> type traits

I needed these meta-programming type traits while working on something else.
Add basic support for these two type traits as well as some tests.
---
 AK/StdLibExtras.h           | 19 +++++++++++++++++++
 AK/Tests/TestTypeTraits.cpp | 17 +++++++++++++++++
 2 files changed, 36 insertions(+)

diff --git a/AK/StdLibExtras.h b/AK/StdLibExtras.h
index 0e6cd49df7..643106aed7 100644
--- a/AK/StdLibExtras.h
+++ b/AK/StdLibExtras.h
@@ -450,6 +450,10 @@ constexpr T exchange(T& slot, U&& value)
     return old_value;
 }
 
+template<typename T>
+struct IsEnum : public IntegralConstant<bool, __is_enum(T)> {
+};
+
 template<typename T>
 struct IsUnion : public IntegralConstant<bool, __is_union(T)> {
 };
@@ -570,6 +574,19 @@ struct IdentityType {
     using Type = T;
 };
 
+template<class T, bool = IsEnum<T>::value>
+struct __UnderlyingType {
+    using Type = __underlying_type(T);
+};
+
+template<class T>
+struct __UnderlyingType<T, false> {
+};
+
+template<class T>
+struct UnderlyingType : __UnderlyingType<T> {
+};
+
 }
 
 using AK::AddConst;
@@ -590,6 +607,7 @@ using AK::IsArithmetic;
 using AK::IsBaseOf;
 using AK::IsClass;
 using AK::IsConst;
+using AK::IsEnum;
 using AK::IsFundamental;
 using AK::IsNullPointer;
 using AK::IsSame;
@@ -605,4 +623,5 @@ using AK::min;
 using AK::move;
 using AK::RemoveConst;
 using AK::swap;
+using AK::UnderlyingType;
 using AK::Void;
diff --git a/AK/Tests/TestTypeTraits.cpp b/AK/Tests/TestTypeTraits.cpp
index 184dabcdc1..c00e6e617e 100644
--- a/AK/Tests/TestTypeTraits.cpp
+++ b/AK/Tests/TestTypeTraits.cpp
@@ -55,6 +55,10 @@
 struct Empty {
 };
 
+enum class Enummer : u8 {
+    Dummmy,
+};
+
 TEST_CASE(FundamentalTypeClassification)
 {
     EXPECT_TRAIT_TRUE(IsVoid, void);
@@ -89,6 +93,12 @@ TEST_CASE(FundamentalTypeClassification)
     EXPECT_TRAIT_FALSE(IsUnsigned, short);
     EXPECT_TRAIT_FALSE(IsUnsigned, char);
     EXPECT_TRAIT_FALSE(IsUnsigned, long);
+
+    EXPECT_TRAIT_TRUE(IsEnum, Enummer);
+    EXPECT_TRAIT_FALSE(IsEnum, Empty);
+    EXPECT_TRAIT_FALSE(IsEnum, int);
+    EXPECT_TRAIT_FALSE(IsEnum, void);
+    EXPECT_TRAIT_FALSE(IsEnum, std::nullptr_t);
 }
 
 TEST_CASE(AddConst)
@@ -101,4 +111,11 @@ TEST_CASE(AddConst)
     EXPECT_EQ_WITH_TRAIT(AddConst, NoConstList, YesConstList);
 }
 
+TEST_CASE(UnderlyingType)
+{
+    using Type = UnderlyingType<Enummer>::Type;
+
+    STATIC_EXPECT_EQ(Type, u8);
+}
+
 TEST_MAIN(TypeTraits)