Specially mark functions coming from wildcard expressions

That way, we can check functions for the ordering requirement as well.
We only have to remember that the last parameter of a wildcard function
does not precede its body (because the parameter is in fact part of the body).

Author: odersky

src/dotty/tools/dotc/ast/Positioned.scala

@@ -171,7 +171,9 @@ abstract class Positioned extends DotClass with Product {
         }
         if (nonOverlapping) {
           this match {
-            case _: Function => // ignore, functions produced from wildcards (e.g. (_ op _) mix parameters and body
+            case _: WildcardFunction
+            if lastPositioned.isInstanceOf[ValDef] && !p.isInstanceOf[ValDef] =>
+              // ignore transition from last wildcard parameter to body
             case _ =>
               assert(!lastPos.exists || !p.pos.exists || lastPos.end <= p.pos.start,
                 s"""position error, child positions overlap or in wrong order
@@ -180,9 +182,9 @@ abstract class Positioned extends DotClass with Product {
                    |1st child position = $lastPos
                    |2nd child          = $p
                    |2nd child position = ${p.pos}""".stripMargin)
-              lastPositioned = p
-              lastPos = p.pos
           }
+          lastPositioned = p
+          lastPos = p.pos
         }
         p.checkPos(nonOverlapping)
       case xs: List[_] =>

src/dotty/tools/dotc/ast/untpd.scala

@@ -46,6 +46,13 @@ object untpd extends Trees.Instance[Untyped] with UntypedTreeInfo {
     override def isTerm = body.isTerm
     override def isType = body.isType
   }
+  /** A function created from a wildcard expression
+   *  @param  placeHolderParams  a list of definitions of synthetic parameters
+   *  @param  body               the function body where wildcards are replaced by
+   *                             references to synthetic parameters.
+   */
+  class WildcardFunction(placeholderParams: List[ValDef], body: Tree) extends Function(placeholderParams, body)
+
   case class InfixOp(left: Tree, op: Name, right: Tree) extends OpTree
   case class PostfixOp(od: Tree, op: Name) extends OpTree
   case class PrefixOp(op: Name, od: Tree) extends OpTree

src/dotty/tools/dotc/parsing/Parsers.scala

@@ -973,7 +973,7 @@ object Parsers {
       else
         try
           if (placeholderParams.isEmpty) t
-          else Function(placeholderParams.reverse, t)
+          else new WildcardFunction(placeholderParams.reverse, t)
         finally placeholderParams = saved
     }