enable ctor's for valid types at compile time
enable ctor's for valid types at compile time

tangrams/variant,edwiincao/variant,daniel-j-h/variant,edwiincao/variant,tempbottle/variant,tempbottle/variant,edwiincao/variant,daniel-j-h/variant,tangrams/variant,tangrams/variant,tempbottle/variant,daniel-j-h/variant