Scala unexpectedly compiles a file, are there implicit conversions at work?

Question

I am trying to figure out why the classes below compile without error in Scala. I would expect the createTestClass method in the DoTest class to fail with a typing error because it tries to supply the implicit parameter which has type TypeClass[A], but instead of such an instance, it provides a lambda with type A => String.

Is there some implicit conversion at work here? How would I go about figuring out what's going on?

object Test {

  trait TypeClass[A] {
    def asString(a: A): String
  }

  object TypeClass {
    def apply[A: TypeClass]: TypeClass[A] = implicitly[TypeClass[A]]
  }

  case class TestClass[A: TypeClass](foo: Option[A] = None)

  object TestClass {
    def apply[A: TypeClass]: TestClass[A] = TestClass[A]()
  }

}

object DoTest {
  import Test.TestClass

  def createTestClass[A]: TestClass[A] =
    TestClass.apply[A]((_: A) => "test")

}

Answer 1

Scala 2.12 introduced a new feature: lambda syntax for SAM (Single Abstract Method) types. That's simply what you're seeing here.