gistfile1.txt

Welcome to Scala 2.12.10 (Java HotSpot(TM) 64-Bit Server VM, Java 1.8.0_201).
Type in expressions for evaluation. Or try :help.

scala> :paste
// Entering paste mode (ctrl-D to finish)

trait Foo

trait NotFoo[-A]
object NotFoo {
  implicit def instance[A]: NotFoo[A] = new NotFoo[A]{}
  implicit val ambiguousFooInstance1: NotFoo[Foo] = new NotFoo[Foo]{}
  implicit val ambiguousFooInstance2: NotFoo[Foo] = new NotFoo[Foo]{}
}

// Exiting paste mode, now interpreting.

defined trait Foo
defined trait NotFoo
defined object NotFoo

scala> def aaa[A: NotFoo](a: A) = a
aaa: [A](a: A)(implicit evidence$1: NotFoo[A])A

scala> aaa(42)
res0: Int = 42

scala> aaa("z")
res1: String = z

scala> aaa(new Foo{})
<console>:14: error: ambiguous implicit values:
 both value ambiguousFooInstance1 in object NotFoo of type => NotFoo[Foo]
 and value ambiguousFooInstance2 in object NotFoo of type => NotFoo[Foo]
 match expected type NotFoo[Foo]
       aaa(new Foo{})
          ^

scala> class FooSubClass extends Foo
defined class FooSubClass

scala> aaa(new FooSubClass)
<console>:14: error: ambiguous implicit values:
 both value ambiguousFooInstance1 in object NotFoo of type => NotFoo[Foo]
 and value ambiguousFooInstance2 in object NotFoo of type => NotFoo[Foo]
 match expected type NotFoo[FooSubClass]
       aaa(new FooSubClass)
          ^