Fix #5234: Add regression test

tests/neg/i5234a.scala

@@ -0,0 +1,10 @@
+object Test {
+  import implicits.Not
+
+  class Foo
+  class Bar
+  implicit def foo: Foo = ???
+  implicitly[Foo]
+  implicitly[Not[Foo]] // error
+  implicitly[Not[Bar]]
+}

tests/neg/i5234b.scala

@@ -0,0 +1,30 @@
+final class Not2[T] private ()
+
+trait LowPriorityNot2 {
+
+  /** A fallback method used to emulate negation in Scala 2 */
+  implicit def default[T]: Not2[T] = Not2.value.asInstanceOf[Not2[T]]
+}
+object Not2 extends LowPriorityNot2 {
+
+  /** A value of type `Not` to signal a successful search for `Not[C]` (i.e. a failing
+   *  search for `C`). A reference to this value will be explicitly constructed by
+   *  Dotty's implicit search algorithm
+   */
+  def value: Not2[Nothing] = new Not2[Nothing]()
+
+  /** One of two ambiguous methods used to emulate negation in Scala 2 */
+  implicit def amb1[T](implicit ev: T): Not2[T] = ???
+
+  /** One of two ambiguous methods used to emulate negation in Scala 2 */
+  implicit def amb2[T](implicit ev: T): Not2[T] = ???
+}
+
+object Test {
+  class Foo
+  class Bar
+  implicit def foo: Foo = ???
+  implicitly[Foo]
+  implicitly[Not2[Foo]] // error
+  implicitly[Not2[Bar]]
+}

tests/neg/i5234c.scala

@@ -0,0 +1,9 @@
+object Test {
+  import implicits.Not
+
+  class Foo
+  implicit def foo: Foo = ???
+
+  def foo[T](implicit ev: Not[T]) = ???
+  foo[Foo] // error
+}