Fix #2456: eliminate syntactic references to Java packages

After typechecking, replace TypeRefs trees of the form Select(p, _) : tpe@TypeRef where p refers to a Java package, by Ident(tpe)

Author: abeln

compiler/src/dotty/tools/dotc/Compiler.scala

@@ -48,7 +48,8 @@ class Compiler {
  List(new sbt.ExtractAPI), // Sends a representation of the API of classes to sbt via callbacks
  List(new Pickler), // Generate TASTY info
  List(new FirstTransform, // Some transformations to put trees into a canonical form
- new CheckReentrant), // Internal use only: Check that compiled program has no data races involving global vars
+ new CheckReentrant, // Internal use only: Check that compiled program has no data races involving global vars
+ new ElimJavaPackages), // Eliminate syntactic references to Java packages
  List(new CheckStatic, // Check restrictions that apply to @static members
  new CheckPhantomCast, // Checks that no Phantom types in are in casts
  new ElimRepeated, // Rewrite vararg parameters and arguments

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

@@ -0,0 +1,46 @@
+package dotty.tools.dotc.transform
+
+import dotty.tools.dotc.ast.tpd._
+import dotty.tools.dotc.core.Contexts.Context
+import dotty.tools.dotc.core.Flags._
+import dotty.tools.dotc.core.Types.{Type, TypeRef}
+import dotty.tools.dotc.transform.TreeTransforms.{MiniPhaseTransform, TransformerInfo}
+
+/**
+ * Eliminates syntactic references to Java packages, so that there's no chance
+ * they accidentally end up in the backend.
+ */
+class ElimJavaPackages extends MiniPhaseTransform {
+
+ override def phaseName: String = "elimJavaPackages"
+
+ override def transformSelect(tree: Select)(implicit ctx: Context, info: TransformerInfo): Tree = {
+ if (isJavaPackage(tree)) {
+ assert(tree.tpe.isInstanceOf[TypeRef], s"Expected tree with type TypeRef, but got ${tree.tpe.show}")
+ Ident(tree.tpe.asInstanceOf[TypeRef])
+ } else {
+ tree
+ }
+ }
+
+ override def checkPostCondition(tree: Tree)(implicit ctx: Context): Unit = {
+ tree match {
+ case tree: Select =>
+ assert(!isJavaPackage(tree), s"Unexpected reference to Java package in ${tree.show}")
+ case _ => ()
+ }
+ }
+
+ /**
+ * Is the given tree a syntactic reference to a Java package?
+ */
+ private def isJavaPackage(tree: Select)(implicit ctx: Context): Boolean = {
+ tree.tpe match {
+ case TypeRef(prefix, _) =>
+ val flags = prefix.termSymbol.flags
+ // Testing for each flag separately is more efficient than using FlagConjunction.
+ flags.is(Package) && flags.is(JavaDefined)
+ case _ => false
+ }
+ }
+}