AK: Add a 'OneOf' concept

Similar to 'SameAs', but for multiple types.

AK/Concepts.h

@@ -36,6 +36,9 @@ concept Enum = IsEnum<T>;
 template<typename T, typename U>
 concept SameAs = IsSame<T, U>;
 
+template<typename U, typename... Ts>
+concept OneOf = IsOneOf<U, Ts...>;
+
 template<typename T, template<typename...> typename S>
 concept SpecializationOf = IsSpecializationOf<T, S>;
 
@@ -117,6 +120,7 @@ using AK::Concepts::Integral;
 using AK::Concepts::IterableContainer;
 using AK::Concepts::IteratorFunction;
 using AK::Concepts::IteratorPairWith;
+using AK::Concepts::OneOf;
 using AK::Concepts::SameAs;
 using AK::Concepts::Signed;
 using AK::Concepts::SpecializationOf;

AK/StdLibExtraDetails.h

@@ -572,6 +572,9 @@ inline constexpr bool IsHashCompatible = false;
 template<typename T>
 inline constexpr bool IsHashCompatible<T, T> = true;
 
+template<typename T, typename... Ts>
+inline constexpr bool IsOneOf = (IsSame<T, Ts> || ...);
+
 }
 using AK::Detail::AddConst;
 using AK::Detail::AddLvalueReference;
@@ -607,6 +610,7 @@ using AK::Detail::IsLvalueReference;
 using AK::Detail::IsMoveAssignable;
 using AK::Detail::IsMoveConstructible;
 using AK::Detail::IsNullPointer;
+using AK::Detail::IsOneOf;
 using AK::Detail::IsPOD;
 using AK::Detail::IsPointer;
 using AK::Detail::IsRvalueReference;