Expand value references to packages to their underlying package objects

A package object can be seen as the facade of a package. For instance, it is the logical
place where we want to write doc comments that explain a package.

So far references to packages cannot be used as values. But if the package has a package
object, it would make sense to allow the package reference with the meaning that it refers
to this object. For instance, let's say we have

```scala
package a
object b
```
Of course, we can use `a.b` as a value. But if we change that to
```scala
package a
package object b
```
we can't anymore. This PR changes that so that we still allow a reference `a.b`
as a value to mean the package object. Due to the way package objects are encoded
the `a.b` reference expands to `a.b.package`.

Author: odersky

Diff for: compiler/src/dotty/tools/dotc/config/Feature.scala

@@ -38,6 +38,7 @@ object Feature:
   val betterMatchTypeExtractors = experimental("betterMatchTypeExtractors")
   val quotedPatternsWithPolymorphicFunctions = experimental("quotedPatternsWithPolymorphicFunctions")
   val betterFors = experimental("betterFors")
+  val packageObjectValues = experimental("packageObjectValues")
 
   def experimentalAutoEnableFeatures(using Context): List[TermName] =
     defn.languageExperimentalFeatures

Diff for: compiler/src/dotty/tools/dotc/reporting/ErrorMessageID.scala

@@ -132,7 +132,7 @@ enum ErrorMessageID(val isActive: Boolean = true) extends java.lang.Enum[ErrorMe
   case MissingCompanionForStaticID // errorNumber: 116
   case PolymorphicMethodMissingTypeInParentID // errorNumber: 117
   case ParamsNoInlineID // errorNumber: 118
-  case JavaSymbolIsNotAValueID // errorNumber: 119
+  case SymbolIsNotAValueID // errorNumber: 119
   case DoubleDefinitionID // errorNumber: 120
   case MatchCaseOnlyNullWarningID // errorNumber: 121
   case ImportedTwiceID // errorNumber: 122

Diff for: compiler/src/dotty/tools/dotc/reporting/messages.scala

@@ -2317,7 +2317,7 @@ class ParamsNoInline(owner: Symbol)(using Context)
   def explain(using Context) = ""
 }
 
-class JavaSymbolIsNotAValue(symbol: Symbol)(using Context) extends TypeMsg(JavaSymbolIsNotAValueID) {
+class SymbolIsNotAValue(symbol: Symbol)(using Context) extends TypeMsg(SymbolIsNotAValueID) {
   def msg(using Context) =
     val kind =
       if symbol is Package then i"$symbol"
@@ -3331,14 +3331,14 @@ final class QuotedTypeMissing(tpe: Type)(using Context) extends StagingMessage(Q
 
   private def witness = defn.QuotedTypeClass.typeRef.appliedTo(tpe)
 
-  override protected def msg(using Context): String = 
+  override protected def msg(using Context): String =
     i"Reference to $tpe within quotes requires a given ${witness} in scope"
 
   override protected def explain(using Context): String =
-    i"""Referencing `$tpe` inside a quoted expression requires a `${witness}` to be in scope. 
+    i"""Referencing `$tpe` inside a quoted expression requires a `${witness}` to be in scope.
         |Since Scala is subject to erasure at runtime, the type information will be missing during the execution of the code.
-        |`${witness}` is therefore needed to carry `$tpe`'s type information into the quoted code. 
-        |Without an implicit `${witness}`, the type `$tpe` cannot be properly referenced within the expression. 
+        |`${witness}` is therefore needed to carry `$tpe`'s type information into the quoted code.
+        |Without an implicit `${witness}`, the type `$tpe` cannot be properly referenced within the expression.
         |To resolve this, ensure that a `${witness}` is available, either through a context-bound or explicitly.
         |"""
 
@@ -3350,7 +3350,7 @@ final class AmbiguousNamedTupleAssignment(key: Name, value: untpd.Tree)(using Co
       |not as an assignment.
       |
       |To assign a value, use curly braces: `{${key} = ${value}}`."""
-  
+
   override protected def explain(using Context): String = ""
 
 class AmbiguousNamedTupleInfixApply()(using Context) extends SyntaxMsg(AmbiguousNamedTupleInfixApplyID):

Diff for: compiler/src/dotty/tools/dotc/transform/Erasure.scala

@@ -24,7 +24,6 @@ import typer.NoChecking
 import inlines.Inlines
 import typer.ProtoTypes.*
 import typer.ErrorReporting.errorTree
-import typer.Checking.checkValue
 import core.TypeErasure.*
 import core.Decorators.*
 import dotty.tools.dotc.ast.{tpd, untpd}
@@ -676,7 +675,7 @@ object Erasure {
       if tree.name == nme.apply && integrateSelect(tree) then
         return typed(tree.qualifier, pt)
 
-      val qual1 = typed(tree.qualifier, AnySelectionProto)
+      var qual1 = typed(tree.qualifier, AnySelectionProto)
 
       def mapOwner(sym: Symbol): Symbol =
         if !sym.exists && tree.name == nme.apply then
@@ -725,7 +724,8 @@ object Erasure {
 
       assert(sym.exists, i"no owner from $owner/${origSym.showLocated} in $tree")
 
-      if owner == defn.ObjectClass then checkValue(qual1)
+      if owner == defn.ObjectClass then
+        qual1 = checkValue(qual1)
 
       def select(qual: Tree, sym: Symbol): Tree =
         untpd.cpy.Select(tree)(qual, sym.name).withType(NamedType(qual.tpe, sym))

Diff for: compiler/src/dotty/tools/dotc/typer/Checking.scala

@@ -781,24 +781,6 @@ object Checking {
           else "Cannot override non-inline parameter with an inline parameter",
           p1.srcPos)
 
-  def checkValue(tree: Tree)(using Context): Unit =
-    val sym = tree.tpe.termSymbol
-    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 match
-          case _: SelectionProto if proto ne SingletonTypeProto => // no value check
-          case _: FunOrPolyProto => // no value check
-          case _ => checkValue(tree)
-      case _ =>
-    tree
-
   /** Check that experimental language imports in `trees`
    *  are done only in experimental scopes. For top-level
    *  experimental imports, all top-level definitions are transformed

Diff for: compiler/src/dotty/tools/dotc/typer/Typer.scala

@@ -609,10 +609,8 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
     // Shortcut for the root package, this is not just a performance
     // optimization, it also avoids forcing imports thus potentially avoiding
     // cyclic references.
-    if (name == nme.ROOTPKG)
-      val tree2 = tree.withType(defn.RootPackage.termRef)
-      checkLegalValue(tree2, pt)
-      return tree2
+    if name == nme.ROOTPKG then
+      return checkLegalValue(tree.withType(defn.RootPackage.termRef), pt)
 
     val rawType =
       val saved1 = unimported
@@ -672,9 +670,7 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
             cpy.Ident(tree)(tree.name.unmangleClassName).withType(checkedType)
           else
             tree.withType(checkedType)
-      val tree2 = toNotNullTermRef(tree1, pt)
-      checkLegalValue(tree2, pt)
-      tree2
+      checkLegalValue(toNotNullTermRef(tree1, pt), pt)
 
     def isLocalExtensionMethodRef: Boolean = rawType match
       case rawType: TermRef =>
@@ -714,21 +710,47 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
       errorTree(tree, MissingIdent(tree, kind, name, pt))
   end typedIdent
 
+  def checkValue(tree: Tree)(using Context): Tree =
+    val sym = tree.tpe.termSymbol
+    if sym.isNoValue && !ctx.isJava then
+      if sym.is(Package)
+          && Feature.enabled(Feature.packageObjectValues)
+          && tree.tpe.member(nme.PACKAGE).hasAltWith(_.symbol.isPackageObject)
+      then
+        typed(untpd.Select(untpd.TypedSplice(tree), nme.PACKAGE))
+      else
+        report.error(SymbolIsNotAValue(sym), tree.srcPos)
+        tree
+    else tree
+
+  /** 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 =
+    tree match
+      case tree: RefTree if tree.name.isTermName =>
+        proto match
+          case _: SelectionProto if proto ne SingletonTypeProto => tree // no value check
+          case _: FunOrPolyProto => tree // no value check
+          case _ => checkValue(tree)
+      case _ => tree
+
   /** (1) If this reference is neither applied nor selected, check that it does
    *      not refer to a package or Java companion object.
    *  (2) Check that a stable identifier pattern is indeed stable (SLS 8.1.5)
    */
-  private def checkLegalValue(tree: Tree, pt: Type)(using Context): Unit =
-    checkValue(tree, pt)
+  private def checkLegalValue(tree: Tree, pt: Type)(using Context): Tree =
+    val tree1 = checkValue(tree, pt)
     if ctx.mode.is(Mode.Pattern)
-       && !tree.isType
+       && !tree1.isType
        && !pt.isInstanceOf[ApplyingProto]
-       && !tree.tpe.match
+       && !tree1.tpe.match
         case tp: NamedType => tp.denot.hasAltWith(_.symbol.isStableMember && tp.prefix.isStable || tp.info.isStable)
         case tp            => tp.isStable
-       && !isWildcardArg(tree)
+       && !isWildcardArg(tree1)
     then
-      report.error(StableIdentPattern(tree, pt), tree.srcPos)
+      report.error(StableIdentPattern(tree1, pt), tree1.srcPos)
+    tree1
 
   def typedSelectWithAdapt(tree0: untpd.Select, pt: Type, qual: Tree)(using Context): Tree =
     val selName = tree0.name
@@ -742,8 +764,7 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
       if checkedType.exists then
         val select = toNotNullTermRef(assignType(tree, checkedType), pt)
         if selName.isTypeName then checkStable(qual.tpe, qual.srcPos, "type prefix")
-        checkLegalValue(select, pt)
-        ConstFold(select)
+        ConstFold(checkLegalValue(select, pt))
       else EmptyTree
 
     // Otherwise, simplify `m.apply(...)` to `m(...)`

Diff for: library/src/scala/runtime/stdLibPatches/language.scala

@@ -141,6 +141,11 @@ object language:
      */
     @compileTimeOnly("`betterFors` can only be used at compile time in import statements")
     object betterFors
+
+    /** Experimental support for package object values
+     */
+    @compileTimeOnly("`packageObjectValues` can only be used at compile time in import statements")
+    object packageObjectValues
   end experimental
 
   /** The deprecated object contains features that are no longer officially suypported in Scala.

Diff for: tests/neg/i21696.check

@@ -5,9 +5,9 @@
   |---------------------------------------------------------------------------------------------------------------------
   | Explanation (enabled by `-explain`)
   |- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
-  | Referencing `T` inside a quoted expression requires a `scala.quoted.Type[T]` to be in scope. 
+  | Referencing `T` inside a quoted expression requires a `scala.quoted.Type[T]` to be in scope.
   | Since Scala is subject to erasure at runtime, the type information will be missing during the execution of the code.
-  | `scala.quoted.Type[T]` is therefore needed to carry `T`'s type information into the quoted code. 
-  | Without an implicit `scala.quoted.Type[T]`, the type `T` cannot be properly referenced within the expression. 
+  | `scala.quoted.Type[T]` is therefore needed to carry `T`'s type information into the quoted code.
+  | Without an implicit `scala.quoted.Type[T]`, the type `T` cannot be properly referenced within the expression.
   | To resolve this, ensure that a `scala.quoted.Type[T]` is available, either through a context-bound or explicitly.
    ---------------------------------------------------------------------------------------------------------------------

Diff for: tests/run/pkgobjvals.check

@@ -0,0 +1,4 @@
+Foo was created
+Foo was created
+Foo was created
+Foo was created

Diff for: tests/run/pkgobjvals.scala

@@ -0,0 +1,22 @@
+import language.experimental.packageObjectValues
+
+package a:
+  package object b:
+    class Foo:
+      println("Foo was created")
+
+    def foo() = Foo()
+  end b
+
+  def test =
+    val bb = b
+    bb.foo()
+    new bb.Foo()
+end a
+
+@main def Test =
+  a.test
+  val ab: a.b.type = a.b
+  ab.foo()
+  new ab.Foo()
+