Add a type cast to RCT_ENUM_CONVERTER for C++ compatibility

Summary:
C++ doesn't provide an implicit cast to an enum value from the enum's backing type. When a `.mm` file calls `RCT_ENUM_CONVERTER`, we end up with the following compiler error:
> Error: cannot initialize return object of type `<TypeName>` with an rvalue of type `NSInteger`

Since `RCT_ENUM_CONVERTER` is a macro, this error will appear whenever we try to expand the macro in a C++ context.

The project compiles and runs as expected when this additional cast is added 😃
Closes #14408

Reviewed By: javache

Differential Revision: D5215584

Pulled By: jballer

fbshipit-source-id: 7f4464afd7cd9dc9a014f646278bae20731f08ba

React/Base/RCTConvert.h

@@ -207,6 +207,17 @@ RCT_CUSTOM_CONVERTER(type, name, [json getter])
 #define RCT_NUMBER_CONVERTER(type, getter) \
 RCT_CUSTOM_CONVERTER(type, type, [RCT_DEBUG ? [self NSNumber:json] : json getter])
 
+/**
+ * When using RCT_ENUM_CONVERTER in ObjC, the compiler is OK with us returning
+ * the underlying NSInteger/NSUInteger. In ObjC++, this is a type mismatch and
+ * we need to explicitly cast the return value to expected enum return type.
+ */
+#ifdef __cplusplus
+#define _RCT_CAST(type, expr) static_cast<type>(expr)
+#else
+#define _RCT_CAST(type, expr) expr
+#endif
+
 /**
  * This macro is used for creating converters for enum types.
  */
@@ -218,7 +229,7 @@ RCT_CUSTOM_CONVERTER(type, type, [RCT_DEBUG ? [self NSNumber:json] : json getter
   dispatch_once(&onceToken, ^{                            \
     mapping = values;                                     \
   });                                                     \
-  return [RCTConvertEnumValue(#type, mapping, @(default), json) getter]; \
+  return _RCT_CAST(type, [RCTConvertEnumValue(#type, mapping, @(default), json) getter]); \
 }
 
 /**
@@ -233,7 +244,7 @@ RCT_CUSTOM_CONVERTER(type, type, [RCT_DEBUG ? [self NSNumber:json] : json getter
   dispatch_once(&onceToken, ^{                            \
     mapping = values;                                     \
   });                                                     \
-  return [RCTConvertMultiEnumValue(#type, mapping, @(default), json) getter]; \
+  return _RCT_CAST(type, [RCTConvertMultiEnumValue(#type, mapping, @(default), json) getter]); \
 }
 
 /**