Fix ElimStaticThis#transformIdent

- Only transform static methods which are inside module classes.
- Make sure that the prefix of the underlying type of the Ident
  is a ThisType of the current module class. For example in
  "scala.Int.box(42)", "box" is an Ident whose underlying type is
  "TermRef(ThisType(TypeRef(ThisType(TypeRef(NoPrefix,scala)),Int$)),box)",
  but we should not trigger an assertion in this case.

Author: smarter

src/dotty/tools/dotc/Compiler.scala

@@ -67,8 +67,8 @@ class Compiler {
            new Constructors,
            new FunctionalInterfaces),
       List(new LambdaLift,   // in this mini-phase block scopes are incorrect. No phases that rely on scopes should be here
-           new ElimStaticThis,
            new Flatten,
+           new ElimStaticThis,
            new RestoreScopes),
       List(/*new PrivateToStatic,*/
            new ExpandPrivate,

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

@@ -9,6 +9,7 @@ import dotty.tools.dotc.core.StdNames._
 import dotty.tools.dotc.core.SymDenotations.SymDenotation
 import TreeTransforms.{MiniPhaseTransform, TransformerInfo}
 import dotty.tools.dotc.core.Types.{ThisType, TermRef}
+import Phases.Phase
 
 /** Replace This references to module classes  in static methods by global identifiers to the
  *  corresponding modules.
@@ -17,6 +18,8 @@ class ElimStaticThis extends MiniPhaseTransform {
   import ast.tpd._
   def phaseName: String = "elimStaticThis"
 
+  override def runsAfter: Set[Class[_ <: Phase]] = Set(classOf[Flatten])
+
   override def transformThis(tree: This)(implicit ctx: Context, info: TransformerInfo): Tree =
     if (!tree.symbol.is(Package) && ctx.owner.enclosingMethod.is(JavaStatic)) {
       assert(tree.symbol.is(ModuleClass))
@@ -25,10 +28,12 @@ class ElimStaticThis extends MiniPhaseTransform {
     else tree
 
   override def transformIdent(tree: tpd.Ident)(implicit ctx: Context, info: TransformerInfo): tpd.Tree = {
-    if (ctx.owner.enclosingMethod.is(JavaStatic)) {
+    val meth = ctx.owner.enclosingMethod
+    // We cannot use meth.enclosingClass because it skips other static classes,
+    // so instead we require this phase to run after Flatten and use meth.owner
+    if (meth.is(JavaStatic) && meth.owner.is(ModuleClass)) {
       tree.tpe match {
-        case TermRef(thiz: ThisType, _) =>
-          assert(thiz.underlying.typeSymbol.is(ModuleClass))
+        case TermRef(thiz: ThisType, _) if (thiz.underlying.typeSymbol == meth.owner) =>
           ref(thiz.underlying.typeSymbol.sourceModule).select(tree.symbol)
         case _ => tree
       }