Add regression tests

Closes #19749
Closes #16208

Author: EugeneFlesselle

tests/pos/i16208.scala

@@ -0,0 +1,12 @@
+
+class Ann(x: Any) extends annotation.Annotation
+object Message:
+  implicit def toNoExplanation(str: String): Message @Ann(str) = ???
+class Message
+
+object report:
+  def error(x: Message): Unit = ???
+
+def test =
+  report.error("a") // works
+  report.error("a".stripMargin) // was an error

tests/pos/i19749.scala

@@ -0,0 +1,15 @@
+import scala.deriving.Mirror
+
+case class A(x: Int, y: String)
+
+trait SomeTrait[T]
+
+object SomeTrait:
+  given [T]: SomeTrait[T] with {}
+
+def f1[T](using p: Mirror.ProductOf[T]): Tuple.Elem[p.MirroredElemTypes, 0] = ???
+
+def f2[T, R](f: T => R)(using SomeTrait[R]) = ???
+
+// Scala3.3 is fine, 3.4 has compilation errors, p MirroredElemTypes type is missing and has been changed to Nothing
+val x = f2(_ => f1[A])