Deal with problem solved by ExplicitSelf in another way

ExplicitSelf was not redundant, after all. The run test i789.scala broke.

But we still cannot leave it in has ExplicitSelf turns stable paths
into unstable ones, which is a problem.

This commit provides another fix to the problem underlying #789.

Author: odersky

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

@@ -331,7 +331,18 @@ object Erasure extends TypeTestsCasts{
      *      e.m -> e.[]m                if `m` is an array operation other than `clone`.
      */
     override def typedSelect(tree: untpd.Select, pt: Type)(implicit ctx: Context): Tree = {
-      val sym = tree.symbol
+      val sym = tree.typeOpt match {
+        case tpe: TermRefWithSignature if tpe.prefix.isInstanceOf[ThisType] =>
+          // It is surprisingly tricky to find out what symbol this tree had before
+          // erasure. The problem is caching. if we compute `this.f` after erasure
+          // we might get the wrong symbol, since `this` does not have an underlying
+          // self type anymore after erasure. But if we then look for the symbol of
+          // that same type before erasure, the cache will give us the same, incorrect
+          // symbol! To sidestep this problem we load the denotation explicitly before
+          // erasure. Test case is i789.scala.
+          tpe.loadDenot(ctx.withPhase(ctx.erasurePhase)).symbol
+        case _ => tree.symbol
+      }
       assert(sym.exists, tree.show)
 
       def select(qual: Tree, sym: Symbol): Tree = {