Question

How can expectedException be declared so that only Exceptions and subclasses can be passed? At present I am using expectedException: Any.

Details.

I have a test utility method that is invoked like this,

assertExceptionThrown("En-passant should be rejected when the previous move was not a double advance", classOf[UnreachablePositionException] ) {
  e.rejectIllegalMove(EnPassant("e5", "d6"))
}

The second argument of the first parameter list is classOf[SomeException]. This the signature of the test method,

  // TODO: Restrict expectedException to Exception or subclass
  def assertExceptionThrown(assertion: String, expectedException: Any)(b:  => Unit) {

My question is how can expectedException be declared so that only Exceptions and subclasses can be passed? At present I am using expectedException: Any.

The full source of the Test trait is here,
https://github.com/janekdb/stair-chess/blob/master/src/test/Test.scala

Answer 1

Use generics:

def assertExceptionThrown[T <: SomeException](assertion: String, expectedException: Class[T])(b:  => Unit)

This says that the type T from expectedException must be SomeException or a subclass thereof.

A demonstration:

class A
class B extends A
def f[T <: A](x: Class[T]) {}   // f accepts Class[A] or a Class[subclass of A]

f(classOf[A])   // fine
f(classOf[B])   // fine
f(classOf[Int]) // error: inferred type arguments [Int] do not conform to method f's type parameter bounds [T <: A]