CheckStatic: do not eliminate non-pure expressions.

Implemented by checking that tree is allowed to access the static member
and all the members on the path to it. Needed as typer has a tendency
to desugar calls into series of selections&calls to This.

Author: DarkDimius

src/dotty/tools/dotc/transform/CheckStatic.scala

@@ -36,7 +36,7 @@ import TypeUtils._
 class CheckStatic extends MiniPhaseTransform { thisTransformer =>
   import ast.tpd._
 
-  override def phaseName = "elimRepeated"
+  override def phaseName = "checkStatic"
 
 
   def check(tree: tpd.DefTree)(implicit ctx: Context) = {
@@ -76,8 +76,19 @@ class CheckStatic extends MiniPhaseTransform { thisTransformer =>
   }
 
   override def transformSelect(tree: tpd.Select)(implicit ctx: Context, info: TransformerInfo): tpd.Tree = {
-    if (tree.symbol.hasAnnotation(defn.ScalaStaticAnnot))
-      ref(tree.symbol)
-    else tree
+    if (tree.symbol.hasAnnotation(defn.ScalaStaticAnnot)) {
+      val symbolWhitelist = tree.symbol.ownersIterator.flatMap(x => if (x.is(Flags.Module)) List(x, x.companionModule) else List(x)).toSet
+      def isSafeQual(t: Tree): Boolean = { // follow the desugared paths created by typer
+        t match {
+          case t: This => true
+          case t: Select => isSafeQual(t.qualifier) && symbolWhitelist.contains(t.symbol)
+          case t: Ident => symbolWhitelist.contains(t.symbol)
+          case t: Block => t.stats.forall(tpd.isPureExpr) && isSafeQual(t.expr)
+        }
+      }
+      if (isSafeQual(tree.qualifier))
+        ref(tree.symbol)
+      else tree
+    } else tree
   }
 }