Fix SCL-23198: Direct references to package objects should be allowed in .mill files

scala/scala-impl/src/org/jetbrains/plugins/scala/lang/psi/impl/expr/ScReferenceExpressionImpl.scala

@@ -195,7 +195,10 @@ class ScReferenceExpressionImpl(node: ASTNode) extends ScReferenceImpl(node) wit
       case inf: ScInfixExpr if this == inf.operation || this == inf.getBaseExpr =>
         StdKinds.refExprQualRef
       case _ =>
-        StdKinds.refExprLastRef
+        // Mill files allow direct references to package
+        // objects, even though normal .scala files do not
+        if (this.containingScalaFile.exists(_.isMillFile)) StdKinds.refExprQualRef
+        else StdKinds.refExprLastRef
     }
   }
 

scala/scala-impl/test/org/jetbrains/plugins/scala/annotator/MillHighlightingTests.scala

@@ -0,0 +1,30 @@
+package org.jetbrains.plugins.scala.annotator
+
+/**
+ * Check customized behavior for Mill build files
+ */
+class MillHighlightingTests extends ScalaHighlightingTestBase {
+  //SCL-23198
+  val code = """package foo{
+               |  object `package`
+               |}
+               |
+               |package bar{
+               |  object qux{
+               |    val reference = foo
+               |  }
+               |}
+               |""".stripMargin
+  def testBuildMillFileAllowsPackageReference(): Unit = {
+    scala.Predef.assert(errorsFromScalaCode(code, "build.mill").isEmpty)
+  }
+  def testPackageFileAllowsPackageReference(): Unit = {
+    scala.Predef.assert(errorsFromScalaCode(code, "package.mill").isEmpty)
+  }
+  def testBuildMillScalaFileAllowsPackageReference(): Unit = {
+    scala.Predef.assert(errorsFromScalaCode(code, "build.mill.scala").isEmpty)
+  }
+  def testNormalScalaFileDisAllowsPackageReference(): Unit = {
+    scala.Predef.assert(errorsFromScalaCode(code, "build.scala").nonEmpty)
+  }
+}