Preserve zero-extent positions when setting children positions

If a tree has no position or a zero-extent position, it should be
synthetic. We can preserve this invariant by always setting a
zero-extent position for these trees here.

Author: smarter

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

@@ -72,8 +72,16 @@ abstract class Positioned extends DotClass with Product {
                                             // is known, from left to right.
     def fillIn(ps: List[Positioned], start: Int, end: Int): Unit = ps match {
       case p :: ps1 =>
-        p.setPos(Position(start, end))
-        fillIn(ps1, end, end)
+        // If a tree has no position or a zero-extent position, it should be
+        // synthetic. We can preserve this invariant by always setting a
+        // zero-extent position for these trees here.
+        if (!p.pos.exists || p.pos.isZeroExtent) {
+          p.setPos(Position(start, start))
+          fillIn(ps1, start, end)
+        } else {
+          p.setPos(Position(start, end))
+          fillIn(ps1, end, end)
+        }
       case nil =>
     }
     while (true) {