SI-4827 Corrected positions assigned to constructor's default arg

* Default arguments are always retained on the <init> method (i.e.,
  the class' constructor). Therefore, when the <init> parameters are
  created, we need to use `duplicateAndKeepPositions` to make sure that
  if a default argument is present, its opaque position is retained as
  well. This is necessary because when parameter accessors (i.e.,
  `fieldDefs`) are created, all default arguments are discared ( as you
  can see in the code, the right-hand-side of a `field` is always an
  `EmptyTree`) - see changes in TreeGen.scala

* When constructing the `fieldDefs`, it is important to adapt their
  range position to avoid overlappings with the positions of default
  arguments. It is worth noting that updating the field's end position
  to `vd.rhs.pos.start` would be incorrect, because `askTypeAt(pos)`
  could return the incorrect tree when the position is equal to
  `vd.rhs.pos.start` (because two nodes including that point position
  would exist in the tree, and `CompilerControl.locateTree(pos)` would
  return the first tree that includes the passed `pos`). This is why
  `1` is subtracted to `vd.rhs.pos.start`. Alternatively, we could have
  used `vd.tpt.pos.end` with similar results. However the logic would
  have become slightly more complex as we would need to handle the case
  where `vd.tpt` doesn't have a range position (for instance, this can
  happen if `-Yinfer-argument-types` is enabled). Therefore, subtracting
  `1` from `vd.rhs.pos.start` seemed the cleanest solution at the
  moment. - see changes in TreeGen.scala.

* If the synthetic constructor contains trees with an opaque range
  position (see point above), it must have a transparent position.
  This can only happen if the constructor's parameters' positions are
  considered, which is why we are now passing `vparamss1` to
  `wrappingPos` - see changes in TreeGen.scala.

* The derived primary constructor should have a transparent position
  as it may contain trees with an opaque range position. Hence, the
  `primaryCtor` position is considered for computing the position of the
  derived constructor - see change in Typers.scala.

Finally, below follows the printing of the tree for test t4287, which
you should compare with the one attached with the previous commit
message:

```
[[syntax trees at end of typer]] // Foo.scala
[0:63]package [0:0]<empty> {
  [0:37]class Baz extends [9:37][39]scala.AnyRef {
    [10:20]<paramaccessor> private[this] val f: [14]Int = _;
    [14]<stable> <accessor> <paramaccessor> def f: [14]Int = [14][14]Baz.this.f;
    <10:31>def <init>(<10:31>f: [17]<type: [17]scala.Int> = [23:31]B.a): [9]Baz = <10:31>{
      <10:31><10:31><10:31>Baz.super.<init>();
      <10:31>()
    }
  };
  [6]<synthetic> object Baz extends [6][6]AnyRef {
    [6]def <init>(): [9]Baz.type = [6]{
      [6][6][6]Baz.super.<init>();
      [9]()
    };
    [14]<synthetic> def <init>$default$1: [14]Int = [30]B.a
  };
  [39:63]object B extends [48:63][63]scala.AnyRef {
    [63]def <init>(): [48]B.type = [63]{
      [63][63][63]B.super.<init>();
      [48]()
    };
    [52:61]private[this] val a: [56]Int = [60:61]2;
    [56]<stable> <accessor> def a: [56]Int = [56][56]B.this.a
  }
}
```

You should notice that the default arg of `Baz` constructor now has a
range position. And that explains why the associated test now returns
the right tree when asking hyperlinking at the location of the default
argument.

1 files changed, 14 insertions, 6 deletions

diff --git a/src/reflect/scala/reflect/internal/TreeGen.scala b/src/reflect/scala/reflect/internal/TreeGen.scala
index a0bd64f850..4b8f63bf33 100644
--- a/src/reflect/scala/reflect/internal/TreeGen.scala
+++ b/src/reflect/scala/reflect/internal/TreeGen.scala
@@ -341,11 +341,13 @@ abstract class TreeGen extends macros.TreeBuilder {
 
     // create parameters for <init> as synthetic trees.
     var vparamss1 = mmap(vparamss) { vd =>
-      atPos(vd.pos.focus) {
+      val param = atPos(vd.pos.makeTransparent) {
         val mods = Modifiers(vd.mods.flags & (IMPLICIT | DEFAULTPARAM | BYNAMEPARAM) | PARAM | PARAMACCESSOR)
-        ValDef(mods withAnnotations vd.mods.annotations, vd.name, vd.tpt.duplicate, vd.rhs.duplicate)
+        ValDef(mods withAnnotations vd.mods.annotations, vd.name, vd.tpt.duplicate, duplicateAndKeepPositions(vd.rhs))
       }
+      param
     }
+    
     val (edefs, rest) = body span treeInfo.isEarlyDef
     val (evdefs, etdefs) = edefs partition treeInfo.isEarlyValDef
     val gvdefs = evdefs map {
@@ -378,15 +380,21 @@ abstract class TreeGen extends macros.TreeBuilder {
                                          // this means that we don't know what will be the arguments of the super call
                                          // therefore here we emit a dummy which gets populated when the template is named and typechecked
         Some(
-          // TODO: previously this was `wrappingPos(superPos, lvdefs ::: argss.flatten)`
-          // is it going to be a problem that we can no longer include the `argss`?
-          atPos(wrappingPos(superPos, lvdefs)) (
+          atPos(wrappingPos(superPos, lvdefs ::: vparamss1.flatten).makeTransparent) (
             DefDef(constrMods, nme.CONSTRUCTOR, List(), vparamss1, TypeTree(), Block(lvdefs ::: List(superCall), Literal(Constant())))))
       }
     }
     constr foreach (ensureNonOverlapping(_, parents ::: gvdefs, focus = false))
     // Field definitions for the class - remove defaults.
-    val fieldDefs = vparamss.flatten map (vd => copyValDef(vd)(mods = vd.mods &~ DEFAULTPARAM, rhs = EmptyTree))
+    
+    val fieldDefs = vparamss.flatten map (vd => {
+      val field = copyValDef(vd)(mods = vd.mods &~ DEFAULTPARAM, rhs = EmptyTree)
+      // Prevent overlapping of `field` end's position with default argument's start position.
+      // This is needed for `Positions.Locator(pos).traverse` to return the correct tree when 
+      // the `pos` is a point position with all its values equal to `vd.rhs.pos.start`.
+      if(field.pos.isRange && vd.rhs.pos.isRange) field.pos = field.pos.withEnd(vd.rhs.pos.start - 1)
+      field
+    })
 
     global.Template(parents, self, gvdefs ::: fieldDefs ::: constr ++: etdefs ::: rest)
   }