Address comments in #5958 (thanks @smarter)

Author: liufengyun

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

@@ -573,10 +573,12 @@ object tpd extends Trees.Instance[Type] with TypedTreeInfo {
  val tree1 = untpdCpy.Block(tree)(stats, expr)
  tree match {
  case tree: Block if (expr.tpe eq tree.expr.tpe) && (expr.tpe eq tree.tpe) =>
- // the second guard is needed in case avoid somehow widened the type.
- // if it did it could potentially need to rewiden it
- // eg {val s = ...; s}
- // changing type of s should change type of block, though type of expr is unchanged - TermRef(s)
+ // The last guard is a conservative check: if `tree.tpe` is different from `expr.tpe`, then
+ // it was computed from widening `expr.tpe`, and tree transforms might cause `expr.tpe.widen`
+ // to change even if `expr.tpe` itself didn't change, e.g:
+ // { val s = ...; s }
+ // If the type of `s` changed, then the type of the block might have changed, even though `expr.tpe`
+ // will still be `TermRef(NoPrefix, s)`
  tree1.withTypeUnchecked(tree.tpe)
  case _ => ta.assignType(tree1, stats, expr)
  }
@@ -587,9 +589,10 @@ object tpd extends Trees.Instance[Type] with TypedTreeInfo {
  tree match {
  case tree: If if (thenp.tpe eq tree.thenp.tpe) && (elsep.tpe eq tree.elsep.tpe) &&
  ((tree.tpe eq thenp.tpe) || (tree.tpe eq elsep.tpe)) =>
- // last guard is needed in case previous if had computed a widened ORType that needs to be recomputed
- // eg {val a = ...; val b = ...; if(...) a else b}
- // changing type of a or b should change type of if, though types of both trees remain unchanged
+ // The last guard is a conservative check similar to the one done in `Block` above,
+ // if `tree.tpe` is not identical to the type of one of its branch, it might have been
+ // computed from the widened type of the branches, so the same reasoning than
+ // in `Block` applies.
  tree1.withTypeUnchecked(tree.tpe)
  case _ => ta.assignType(tree1, thenp, elsep)
  }

compiler/src/dotty/tools/dotc/transform/ElimErasedValueType.scala

@@ -127,15 +127,10 @@ class ElimErasedValueType extends MiniPhase with InfoTransformer {
  override def transformInlined(tree: Inlined)(implicit ctx: Context): Tree =
  transformTypeOfTree(tree)
 
- // FIXME: transformIf and transformBlock won't be required anymore once #444 is fixed.
  override def transformIdent(tree: Ident)(implicit ctx: Context): Tree =
  transformTypeOfTree(tree)
  override def transformSelect(tree: Select)(implicit ctx: Context): Tree =
  transformTypeOfTree(tree)
- override def transformBlock(tree: Block)(implicit ctx: Context): Tree =
- transformTypeOfTree(tree)
- override def transformIf(tree: If)(implicit ctx: Context): Tree =
- transformTypeOfTree(tree)
  override def transformTypeTree(tree: TypeTree)(implicit ctx: Context): Tree =
  transformTypeOfTree(tree)
 }

tests/pos/vcblock.scala

@@ -0,0 +1,6 @@
+class Position(val foo: Int) extends AnyVal {
+ def thing(): Position = {
+ val y = new Position(1)
+ y
+ }
+}

tests/pos/vcif.scala

@@ -0,0 +1,4 @@
+class Position(val foo: Int) extends AnyVal {
+ def orElse(that: Position) =
+ if (this != null) this else that
+}