How to know if an object is an instance of a TypeTag's type?

Question

I have a function which is able to know if an object is an instance of a Manifest's type. I would like to migrate it to a TypeTag version. The old function is the following one:

def myIsInstanceOf[T: Manifest](that: Any) = 
  implicitly[Manifest[T]].erasure.isInstance(that)

I have been experimenting with the TypeTags and now I have this TypeTag version:

// Involved definitions
def myInstanceToTpe[T: TypeTag](x: T) = typeOf[T]
def myIsInstanceOf[T: TypeTag, U: TypeTag](tag: TypeTag[T], that: U) = 
  myInstanceToTpe(that) stat_<:< tag.tpe

// Some invocation examples
class A
class B extends A
class C

myIsInstanceOf(typeTag[A], new A)        /* true */
myIsInstanceOf(typeTag[A], new B)        /* true */
myIsInstanceOf(typeTag[A], new C)        /* false */

Is there any better way to achieve this task? Can the parameterized U be omitted, using an Any instead (just as it is done in the old function)?

Answer 1

If it suffices to use subtyping checks on erased types, do as Travis Brown suggested in the comment above:

def myIsInstanceOf[T: ClassTag](that: Any) =
  classTag[T].runtimeClass.isInstance(that)

Otherwise you need to explicitly spell out the U type, so that scalac captures its type in a type tag:

def myIsInstanceOf[T: TypeTag, U: TypeTag] =
  typeOf[U] <:< typeOf[T]