Fix #4148: Do not emit static forwarders for non-top-level objects by default.

Scala/JVM does not emit static forwarders for non-top-level
objects. The reason we do it in Scala.js is for implementations of
JDK classes that contain static methods in inner static classes
(#3950). However, this causes issues with existing Scala projects
that have non-top-level `object`s and `class`es with names
differing only in case.

In this commit, we now only emit static forwarders for top-level
objects by default, and add an opt-in option to emit them for
non-top-level objects, which should be used by implementations of
JDK classes.

Author: sjrd

compiler/src/main/scala/org/scalajs/nscplugin/GenJSCode.scala

@@ -1008,16 +1008,22 @@ abstract class GenJSCode[G <: Global with Singleton](val global: G)
      * back-end considers them as colliding because they have the same name,
      * but we must not.
      *
-     * Further, unlike scalac, we emit forwarders for *any* static object, not
-     * just top-level ones. This is important so we can implement static methods
+     * By default, we only emit forwarders for top-level objects, like scalac.
+     * However, if requested via a compiler option, we enable them for all
+     * static objects. This is important so we can implement static methods
      * of nested static classes of JDK APIs (see #3950).
      */
 
     /** Is the given Scala class, interface or module class a candidate for
      *  static forwarders?
      */
-    def isCandidateForForwarders(sym: Symbol): Boolean =
-      !settings.noForwarders && sym.isStatic && !isImplClass(sym)
+    def isCandidateForForwarders(sym: Symbol): Boolean = {
+      !settings.noForwarders && sym.isStatic && !isImplClass(sym) && {
+        // Reject non-top-level objects unless opted in via the appropriate option
+        scalaJSOpts.genStaticForwardersForNonTopLevelObjects ||
+        !sym.name.containsChar('$') // this is the same test that scalac performs
+      }
+    }
 
     /** Gen the static forwarders to the members of a class or interface for
      *  methods of its companion object.

compiler/src/main/scala/org/scalajs/nscplugin/ScalaJSOptions.scala

@@ -26,6 +26,14 @@ trait ScalaJSOptions {
    *  If false, bad calls to classOf will cause an error. */
   def fixClassOf: Boolean
 
+  /** Should static forwarders be emitted for non-top-level objects.
+   *
+   *  Scala/JVM does not do that. Since Scala.js 1.2.0, we do not do it by
+   *  default either, but this option can be used to opt in. This is necessary
+   *  for implementations of JDK classes.
+   */
+  def genStaticForwardersForNonTopLevelObjects: Boolean
+
   /** which source locations in source maps should be relativized (or where
    *  should they be mapped to)? */
   def sourceURIMaps: List[URIMap]

compiler/src/main/scala/org/scalajs/nscplugin/ScalaJSPlugin.scala

@@ -55,6 +55,7 @@ class ScalaJSPlugin(val global: Global) extends NscPlugin {
   object scalaJSOpts extends ScalaJSOptions {
     import ScalaJSOptions.URIMap
     var fixClassOf: Boolean = false
+    var genStaticForwardersForNonTopLevelObjects: Boolean = false
     lazy val sourceURIMaps: List[URIMap] = {
       if (_sourceURIMaps.nonEmpty)
         _sourceURIMaps.reverse
@@ -118,6 +119,8 @@ class ScalaJSPlugin(val global: Global) extends NscPlugin {
     for (option <- options) {
       if (option == "fixClassOf") {
         fixClassOf = true
+      } else if (option == "genStaticForwardersForNonTopLevelObjects") {
+        genStaticForwardersForNonTopLevelObjects = true
       } else if (option.startsWith("mapSourceURI:")) {
         val uris = option.stripPrefix("mapSourceURI:").split("->")
 
@@ -170,6 +173,10 @@ class ScalaJSPlugin(val global: Global) extends NscPlugin {
       |     - strips away the prefix FROM_URI (if it matches)
       |     - optionally prefixes the TO_URI, where stripping has been performed
       |     - any number of occurrences are allowed. Processing is done on a first match basis.
+      |  -P:$name:genStaticForwardersForNonTopLevelObjects
+      |     Generate static forwarders for non-top-level objects.
+      |     This option should be used by codebases that implement JDK classes.
+      |     When used together with -Xno-forwarders, this option has no effect.
       |  -P:$name:fixClassOf
       |     Repair calls to Predef.classOf that reach Scala.js.
       |     WARNING: This is a tremendous hack! Expect ugly errors if you use this option.

project/Build.scala

@@ -1026,6 +1026,8 @@ object Build {
        * flag prevents these mistakes from happening.
        */
       scalacOptions += "-Yno-predef",
+      // We implement JDK classes, so we emit static forwarders for all static objects
+      scalacOptions += "-P:scalajs:genStaticForwardersForNonTopLevelObjects",
 
       resourceGenerators in Compile += Def.task {
         val output = (resourceManaged in Compile).value / "java/lang/Object.sjsir"
@@ -1059,6 +1061,8 @@ object Build {
        * we rely on the Scala library.
        */
       scalacOptions += "-Yno-predef",
+      // We implement JDK classes, so we emit static forwarders for all static objects
+      scalacOptions += "-P:scalajs:genStaticForwardersForNonTopLevelObjects",
 
       headerSources in Compile ~= { srcs =>
         srcs.filter { src =>
@@ -1706,6 +1710,8 @@ object Build {
             moduleKind == ModuleKind.ESModule) // this is an approximation that works for now
       },
 
+      scalacOptions in Test += "-P:scalajs:genStaticForwardersForNonTopLevelObjects",
+
       scalaJSLinkerConfig ~= { _.withSemantics(TestSuiteLinkerOptions.semantics _) },
       scalaJSModuleInitializers in Test ++= TestSuiteLinkerOptions.moduleInitializers,