Add neg test to show it no longer crashes

Author: odersky

tests/neg/i3703.scala

@@ -0,0 +1,11 @@
+package bar {
+  trait M[F[_]]
+  class S[XS[_] <: M[XS], A](val x: XS[A])
+  object S {
+    def apply[X[_] <: M[X], A](x: X[A]): S[X, A] = S[X, A](x)
+    def unapply[X[_] <: M[X], A](p: S[X, A]): S[X, A] = S(p.x)  // error, scalac allows this but dotty fails to infer the type arguments.
+  }
+}
+
+
+