Add tests with dependencies from second type parameter clause

Add tests with dependencies from second type parameter clause to
first type parameter and extension parameter clauses.

Author: odersky

tests/pos/extmethods.scala

@@ -24,6 +24,24 @@ object CollectionStrawMan {
 extension [A](xs: List[A])
   inline def foldl[B](z: B)(op: (B, A) => B): B =
     (xs: List[A]).foldLeft(z)(op)
+  inline def concat[B <: A](ys: List[B]): List[A] = xs ++ ys
 
 val x = List("a", "b").foldl[Int](0)((x, y) => x + y.length)
+val y = Nil.concat(1 :: Nil)
+val y1: List[Int] = y
+val z = (1 :: Nil).concat(Nil)
+val z1: List[Int] = z
 
+trait TT:
+  type A
+  val m: A
+  def f[B <: A](x: B): A = if ??? then m else x
+
+extension (x: TT)
+  def foo[B <: x.A](y: B) = x.f(y)
+
+object CC extends TT:
+  type A = Seq[Int]
+  val m = Nil
+
+val xx = CC.foo(List(1, 2, 3))