Handle positions in extension methods

1. Take extension methods into account when checking positions.

2. Fix position of empty type tree in method definition

Without this measure, the empty type tree gets the end position of
the preceding element. For an extension method with type parameters
this is the extension parameter list, but it should be the type parameters.
Fixing the position explicitly fixes the problem.

Author: odersky

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

@@ -4,7 +4,7 @@ package ast
 import util.Positions._
 import core.Contexts.Context
 import core.Decorators._
-import core.Flags.JavaDefined
+import core.Flags.{JavaDefined, Extension}
 import core.StdNames.nme
 
 /** A base class for things that have positions (currently: modifiers and trees)
@@ -208,6 +208,12 @@ abstract class Positioned extends Product {
         // Leave out tparams, they are copied with wrong positions from parent class
         check(tree.mods)
         check(tree.vparamss)
+      case tree: DefDef if tree.mods.is(Extension) && tree.vparamss.nonEmpty =>
+        check(tree.vparamss.head)
+        check(tree.tparams)
+        check(tree.vparamss.tail)
+        check(tree.tpt)
+        check(tree.rhs)
       case _ =>
         val end = productArity
         var n = 0

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

@@ -1022,7 +1022,7 @@ object Parsers {
 
     def typedOpt(): Tree =
       if (in.token == COLON) { in.nextToken(); toplevelTyp() }
-      else TypeTree()
+      else TypeTree().withPos(Position(in.lastOffset))
 
     def typeDependingOn(location: Location.Value): Tree =
       if (location == Location.InParens) typ()