SI-7731 make CannotHaveAttrs more consistent

Previously setPos, pos_=, setType, tpe_= all behaved inconsistently
between each other even though they all represent similar
attributes that cannot be changed on CannotHaveAttrs trees.

In order to simplify handling of such trees in compiler code each
of these fields now supports assignment to its current default
value: NoType for tpe and NoPosition for pos. Such assignments
don't mutate underlying trees.

1 files changed, 6 insertions, 4 deletions

diff --git a/src/reflect/scala/reflect/internal/Trees.scala b/src/reflect/scala/reflect/internal/Trees.scala
index df1ba1e2ea..02bee5e369 100644
--- a/src/reflect/scala/reflect/internal/Trees.scala
+++ b/src/reflect/scala/reflect/internal/Trees.scala
@@ -1017,14 +1017,16 @@ trait Trees extends api.Trees { self: SymbolTable =>
   trait CannotHaveAttrs extends Tree {
     override def canHaveAttrs = false
 
-    private def unsupported(what: String, args: Any*) =
-      throw new UnsupportedOperationException(s"$what($args) inapplicable for "+self.toString)
+    private def requireLegal(value: Any, allowed: Any, what: String) =
+      require(value == allowed, s"can't set $what for $self to value other than $allowed")
 
     super.setPos(NoPosition)
-    override def setPos(pos: Position) = unsupported("setPos", pos)
+    override def setPos(pos: Position) = { requireLegal(pos, NoPosition, "pos"); this }
+    override def pos_=(pos: Position) = setPos(pos)
 
     super.setType(NoType)
-    override def tpe_=(t: Type) = if (t != NoType) unsupported("tpe_=", t)
+    override def setType(t: Type) = { requireLegal(t, NoType, "tpe"); this }
+    override def tpe_=(t: Type) = setType(t)
   }
 
   case object EmptyTree extends TermTree with CannotHaveAttrs { override def isEmpty = true; val asList = List(this) }