Fix refs to inner objects

A reference to an object from anywhere in its module class can be
established by the This of the module class. The previous behavior always
referenced the object as a term ref which might cause a reference to
the outer This which might not be available (since this is not tracked
by ExplicitOuter). Brings t3174.scala back from disabled.

Author: odersky

src/dotty/tools/dotc/ast/tpd.scala

@@ -326,23 +326,24 @@ object tpd extends Trees.Instance[Type] with TypedTreeInfo {
   def ref(tp: NamedType)(implicit ctx: Context): Tree =
     if (tp.isType) TypeTree(tp)
     else if (prefixIsElidable(tp)) Ident(tp)
+    else if (tp.symbol.is(Module) && ctx.owner.isContainedIn(tp.symbol.moduleClass))
+      correctForPath(This(tp.symbol.moduleClass.asClass))
     else tp.prefix match {
-      case pre: SingletonType =>
-        val prefix =
-          singleton(pre) match {
-            case t: This if ctx.erasedTypes && !(t.symbol == ctx.owner.enclosingClass || t.symbol.isStaticOwner) =>
-              // after erasure outer paths should be respected
-              new ExplicitOuter.OuterOps(ctx).path(t.tpe.widen.classSymbol)
-            case t =>
-              t
-          }
-        prefix.select(tp)
+      case pre: SingletonType => correctForPath(singleton(pre)).select(tp)
       case pre => SelectFromTypeTree(TypeTree(pre), tp)
     } // no checks necessary
 
   def ref(sym: Symbol)(implicit ctx: Context): Tree =
     ref(NamedType(sym.owner.thisType, sym.name, sym.denot))
 
+  private def correctForPath(t: Tree)(implicit ctx: Context) = t match {
+    case t: This if ctx.erasedTypes && !(t.symbol == ctx.owner.enclosingClass || t.symbol.isStaticOwner) =>
+      // after erasure outer paths should be respected
+      new ExplicitOuter.OuterOps(ctx).path(t.tpe.widen.classSymbol)
+    case t =>
+      t
+  }
+
   def singleton(tp: Type)(implicit ctx: Context): Tree = tp match {
     case tp: TermRef => ref(tp)
     case tp: ThisType => This(tp.cls)

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

@@ -221,7 +221,7 @@ object ExplicitOuter {
       case nw: New =>
         isOuter(nw.tpe.classSymbol.owner.enclosingClass)
       case _ =>
-       false
+        false
     }
   }
 

tests/disabled/pos/t3174.scala renamed to tests/pos/t3174.scala

@@ -2,7 +2,9 @@ object test {
   def method(): Unit = {
     class Foo extends AnyRef {
       object Color {
-        object Blue
+        object Blue {
+          //val b = new Board
+        }
       }
 
       class Board {