Address review comments

Author: odersky

compiler/src/dotty/tools/dotc/ast/Desugar.scala

@@ -914,7 +914,7 @@ object desugar {
           ctx.error(em"No extension method $allowed", mdef.sourcePos)
           mdef
         }
-        else cpy.DefDef(mdef)(tparams = tparams ++ mdef.tparams, vparamss = leadingParams :: Nil)
+        else cpy.DefDef(mdef)(tparams = tparams ++ mdef.tparams, vparamss = leadingParams :: mdef.vparamss)
           .withFlags(Extension)
       case mdef: Import =>
         mdef

tests/neg/extension-methods.scala

@@ -10,5 +10,9 @@ object Test {
   "".l2 // error
   1.l1 // error
 
-
+  given [T](xs: List[T]) {
+    def (x: Int) f1: T = ???  // error: No extension method allowed here, since collective parameters are given
+    def f2[T]: T = ???        // error: T is already defined as type T
+    def f3(xs: List[T]) = ??? // error: xs is already defined as value xs
+  }
 }

tests/run/extmethods2.scala

@@ -19,10 +19,14 @@ object Test extends App {
     given ListOps[T](xs: List[T]) {
       def second: T = xs.tail.head
       def third: T = xs.tail.tail.head
+      def concat(ys: List[T]) = xs ++ ys
+      def zipp[U](ys: List[U]): List[(T, U)] = xs.zip(ys)
     }
     given (xs: List[Int]) {
       def prod = (1 /: xs)(_ * _)
     }
+
+
   }
 
   object B {
@@ -33,6 +37,9 @@ object Test extends App {
     assert(A.ListOps.second[Int](xs) == 2)
     assert(A.ListOps.third(xs) == 3)
     assert(xs.prod == 6)
+    assert(xs.concat(xs).length == 6)
+    assert(xs.zipp(xs).map(_ + _).prod == 36)
+    assert(xs.zipp[Int, Int](xs).map(_ + _).prod == 36)
   }
 }