fix #1648: don't define companion object for java.lang.Object

Author: liufengyun

src/dotty/tools/dotc/core/SymbolLoaders.scala

@@ -86,18 +86,27 @@ class SymbolLoaders {
       (module, modcls) => new PackageLoader(module, pkg)).entered
   }
 
+  /** Whether the class to be loaded is `java.lang.Object`
+   *
+   *  `java.lang.Object` should never be loaded from classfiles
+   */
+  def isObjectClass(owner: Symbol, name: PreName)(implicit ctx: Context): Boolean =
+    owner.fullName.toString == "java.lang" && name.show == "Object"
+
   /** Enter class and module with given `name` into scope of `owner`
    *  and give them `completer` as type.
    */
   def enterClassAndModule(
       owner: Symbol, name: PreName, completer: SymbolLoader,
       flags: FlagSet = EmptyFlags, scope: Scope = EmptyScope)(implicit ctx: Context): Unit = {
     val clazz = enterClass(owner, name, completer, flags, scope)
-    val module = enterModule(
-      owner, name, completer,
-      modFlags = flags.toTermFlags & RetainedModuleValFlags,
-      clsFlags = flags.toTypeFlags & RetainedModuleClassFlags,
-      scope = scope)
+
+    if (!isObjectClass(owner, name))
+      enterModule(
+        owner, name, completer,
+        modFlags = flags.toTermFlags & RetainedModuleValFlags,
+        clsFlags = flags.toTypeFlags & RetainedModuleClassFlags,
+        scope = scope)
   }
 
   /** In batch mode: Enter class and module with given `name` into scope of `owner`

tests/neg/i1648.scala

@@ -0,0 +1 @@
+class Foo { Object[A] } // error error