Add a way to get type at index for variant

Summary: This diff adds possibility to extract the type of the nth element of the `CK::Variant`. Unit tests contain the example of extracting the types

Reviewed By: Andrey-Mishanin

Differential Revision: D24046072

fbshipit-source-id: 5a414929cb543407e5da64b144c90c3413765c6e

Author: Oleksii Dykan

ComponentKitTests/CKVariantTests.mm

@@ -206,4 +206,15 @@ - (void)test_FunctionPointer
   XCTAssertEqual(v, 43);
 }
 
+- (void)test_GettingTypeAtIndex
+{
+  Variant<int, std::string>::AlternativeAt<0> intVar = 3;
+  XCTAssertEqual(intVar, 3);
+  
+  Variant<int, std::string>::AlternativeAt<1> stringVar = "test";
+  XCTAssertEqual(stringVar, "test");
+  
+  // Variant<int, std::string>::AlternativeAt<2> invalidVar; this one fails at compile time
+}
+
 @end

RenderCore/Base/CKVariant.h

@@ -492,6 +492,15 @@ class Variant : private VariantDetail::VariantStorage<Types...>,
   // Inherit assignment operators from each of the types in Types
   using VariantDetail::VariantChoice<sizeof...(Types), Types...>::operator=;
 
+  /**
+   Provides compile-time indexed access to the types of the variant's alternatives
+   \code
+   using T = CK::Variant<bool, short, int>::AlternativeAt<1>; // T == short
+   \endcode
+   */
+  template <unsigned i>
+  using AlternativeAt = VariantDetail::NthElement<VariantDetail::Typelist<Types...>, i>;
+
   Variant() = default;
 
   Variant(const Variant& other) {