Add test

Author: odersky

tests/generic-java-signatures/i3653.check

@@ -1 +1,2 @@
 public <T> scala.FunctionXXL Foo.bar()
+public scala.Function1<scala.Function1<java.lang.String, scala.runtime.BoxedUnit>, scala.runtime.BoxedUnit> Foo.baz()

tests/generic-java-signatures/i3653.scala

@@ -3,11 +3,16 @@ class Foo {
     (a0: T, a1: T, a2: T, a3: T, a4: T, a5: T, a6: T, a7: T, a8: T, a9: T,
      b0: T, b1: T, b2: T, b3: T, b4: T, b5: T, b6: T, b7: T, b8: T, b9: T,
      c0: T, c1: T, c2: T, c3: T, c4: T, c5: T, c6: T, c7: T, c8: T, c9: T) => 0
+
+  // #6946
+  def baz = (x: given String => Unit) => x given ""
 }
 
 object Test {
   def main(args: Array[String]): Unit = {
-    val meth = classOf[Foo].getDeclaredMethod("bar")
-    println(meth.toGenericString)
+    val meth1 = classOf[Foo].getDeclaredMethod("bar")
+    println(meth1.toGenericString)
+    val meth2 = classOf[Foo].getDeclaredMethod("baz")
+    println(meth2.toGenericString)
   }
 }