Scala.js: Fix classOf[Null] and classOf[Nothing].

For almost all purposes, `Null` and `Nothing` should be erased to
`scala.runtime.{Null$,Nothing$}`, so we directly change
`encodeClassName` to do that.

The only place where they should be encoded as the special IR
TypeRefs `NullRef` and `NothingRef` is in param and result types of
`MethodName`s, which was already taken care of by
`paramOrResultTypeRef` but that was adjusted to compensate for the
changes in `encodeClassName`. The new `paramOrResultTypeRef` is now
equivalent to the one in scalac.

Author: sjrd

compiler/src/dotty/tools/backend/sjs/JSEncoding.scala

@@ -43,8 +43,8 @@ import dotty.tools.backend.jvm.DottyBackendInterface.symExtensions
  */
 object JSEncoding {
 
-  private val ScalaNothingClassName = ClassName("scala.Nothing")
-  private val ScalaNullClassName = ClassName("scala.Null")
+  private val ScalaRuntimeNothingClassName = ClassName("scala.runtime.Nothing$")
+  private val ScalaRuntimeNullClassName = ClassName("scala.runtime.Null$")
 
   // Fresh local name generator ----------------------------------------------
 
@@ -208,9 +208,9 @@ object JSEncoding {
   /** Computes the type ref for a type, to be used in a method signature. */
   private def paramOrResultTypeRef(tpe: Type)(using Context): jstpe.TypeRef = {
     toTypeRef(tpe) match {
-      case jstpe.ClassRef(ScalaNullClassName)    => jstpe.NullRef
-      case jstpe.ClassRef(ScalaNothingClassName) => jstpe.NothingRef
-      case otherTypeRef                          => otherTypeRef
+      case jstpe.ClassRef(ScalaRuntimeNullClassName)    => jstpe.NullRef
+      case jstpe.ClassRef(ScalaRuntimeNothingClassName) => jstpe.NothingRef
+      case otherTypeRef                                 => otherTypeRef
     }
   }
 
@@ -243,14 +243,20 @@ object JSEncoding {
       if (sym.isAllOf(ModuleClass | JavaDefined)) sym.linkedClass
       else sym
 
-    if (sym1 == defn.BoxedUnitClass) {
-      /* Rewire scala.runtime.BoxedUnit to java.lang.Void, as the IR expects.
-       * BoxedUnit$ is a JVM artifact.
-       */
+    /* Some rewirings:
+     * - scala.runtime.BoxedUnit to java.lang.Void, as the IR expects.
+     *   BoxedUnit$ is a JVM artifact.
+     * - scala.Nothing to scala.runtime.Nothing$.
+     * - scala.Null to scala.runtime.Null$.
+     */
+    if (sym1 == defn.BoxedUnitClass)
       ir.Names.BoxedUnitClass
-    } else {
+    else if (sym1 == defn.NothingClass)
+      ScalaRuntimeNothingClassName
+    else if (sym1 == defn.NullClass)
+      ScalaRuntimeNullClassName
+    else
       ClassName(sym1.javaClassName)
-    }
   }
 
   def toIRType(tp: Type)(using Context): jstpe.Type = {

project/Build.scala

@@ -1055,7 +1055,7 @@ object Build {
           ++ (dir / "shared/src/test/scala/org/scalajs/testsuite/junit" ** "*.scala").get
           ++ (dir / "shared/src/test/scala/org/scalajs/testsuite/niobuffer" ** "*.scala").get
           ++ (dir / "shared/src/test/scala/org/scalajs/testsuite/niocharset" ** (("*.scala": FileFilter)  -- "BaseCharsetTest.scala" -- "Latin1Test.scala" -- "USASCIITest.scala" -- "UTF16Test.scala" -- "UTF8Test.scala")).get
-          ++ (dir / "shared/src/test/scala/org/scalajs/testsuite/scalalib" ** (("*.scala": FileFilter)  -- "ArrayBuilderTest.scala" -- "ClassTagTest.scala" -- "EnumerationTest.scala" -- "SymbolTest.scala")).get
+          ++ (dir / "shared/src/test/scala/org/scalajs/testsuite/scalalib" ** (("*.scala": FileFilter) -- "EnumerationTest.scala" -- "SymbolTest.scala")).get
           ++ (dir / "shared/src/test/require-sam" ** "*.scala").get
           ++ (dir / "shared/src/test/require-jdk8/org/scalajs/testsuite/compiler" ** (("*.scala": FileFilter) -- "DefaultMethodsTest.scala")).get
           ++ (dir / "shared/src/test/require-jdk8/org/scalajs/testsuite/javalib/lang" ** "*.scala").get