Backport "Disallow taking singleton types of packages again" (#18330)

Backports #18232

Author: Kordyjan

compiler/src/dotty/tools/dotc/typer/Checking.scala

@@ -748,13 +748,16 @@ object Checking {
     if sym.isNoValue && !ctx.isJava then
       report.error(JavaSymbolIsNotAValue(sym), tree.srcPos)
 
+  /** Check that `tree` refers to a value, unless `tree` is selected or applied
+   *  (singleton types x.type don't count as selections).
+   */
   def checkValue(tree: Tree, proto: Type)(using Context): tree.type =
     tree match
-      case tree: RefTree
-      if tree.name.isTermName
-         && !proto.isInstanceOf[SelectionProto]
-         && !proto.isInstanceOf[FunOrPolyProto] =>
-        checkValue(tree)
+      case tree: RefTree if tree.name.isTermName =>
+        proto match
+          case _: SelectionProto if proto ne SingletonTypeProto => // no value check
+          case _: FunOrPolyProto => // no value check
+          case _ => checkValue(tree)
       case _ =>
     tree
 

tests/neg/i18109.scala

@@ -0,0 +1,11 @@
+package foo {}
+
+package bar {
+  object Test {
+    def qux[A] = 123
+    def main(args: Array[String]): Unit = {
+      val y = qux[foo.type] // error
+      val x = valueOf[foo.type] // error
+    }
+  }
+}