Do Scala type lambdas cost of a reflective call?

Question

I'm working with a lot of code with type lambdas at the moment and noticed that IntelliJ is warning me that it's an "Advanced language feature: reflective call".

Sample code:

implicit def monoidApplicative[M](M: Monoid[M]) =
  new Applicative[({ type f[x] = Const[M, x] })#f] {
    def unit[A](a: => A): M = M.zero
    override def apply[A,B](m1: M)(m2: M): M = M.op(m1, m2)
  }

Note: I figure that this could be a bug the IntelliJ Scala plugin as it would make sense that type lambdas are resolved at compile time.

Answer 1

IDEA just gets confused because of the { } block. None of this code even exists at runtime.

Here is a small example. A generic identity method for types with one type argument:

def id[F[_], A](value: F[A]) = value

It is not possible to call this method with a type that takes two type arguments:

class Test[A, B]
id(new Test[Int, Int])

<console>:10: error: type mismatch;
 found   : Test[Int,Int]
 required: F[A]
              id(new Test[Int, Int])

But we can use a type lambda to define a function id2 that is implemented in terms of id:

def id2[F[_, _], A, B](value: F[A, B]) =
  id[({ type f[x] = F[A, x] })#f, B](value)

id2(new Test[Int, Int])
res3: Test[Int,Int] = Test@1a53ac0c

Quite convoluted, but it works. Now let's look at the bytecode.

scala> :javap -c id2

...

  public <F, A, B> F id2(F);
Code:
   0: getstatic     #19                 // Field .MODULE$:L;
   3: aload_1
   4: invokevirtual #22                 // Method .id:(Ljava/lang/Object;)Ljava/lang/Object;
   7: areturn

...

As you can see, there is nothing left of all the type trickery. No reflection, nothing. Just java.lang.Object.