Documentation claims they are supported with forward declaration but it is not the case:
typedef struct
{
EPropertyType MappingType;
if (MappingType != 0xFF)
{
if (MappingType == EnumProperty)
{
Property Inner; // ERROR
}
elsef if (MappingType == EPropertyType::EnumAsByteProperty)
{
}
};
} Property <optimize=false>;