Add restriction on phantom latice definition.

Author: nicolasstucki

compiler/src/dotty/tools/dotc/reporting/diagnostic/ErrorMessageID.java

@@ -64,6 +64,7 @@ public enum ErrorMessageID {
     MatchOnPhantomID,
     IfElsePhantomID,
     PhantomIsInObjectID,
+    PhantomObjectIsInPackageOrObjectID,
     ;
 
     public int errorNumber() {

compiler/src/dotty/tools/dotc/reporting/diagnostic/messages.scala

@@ -1296,4 +1296,12 @@ object messages {
     val explanation =
       hl"""|""".stripMargin
   }
+
+  case class PhantomObjectIsInPackageOrObject()(implicit ctx: Context) extends Message(PhantomObjectIsInPackageOrObjectID) {
+    val kind = "Phantom restriction"
+    val msg = s"An ${"object"} extending ${"scala.Phantom"} must be a top level ${"object"} or in another object"
+
+    val explanation =
+      hl"""|""".stripMargin
+  }
 }

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

@@ -1358,8 +1358,12 @@ class Typer extends Namer with TypeAssigner with Applications with Implicits wit
           cls, isRequired, cdef.pos)
     }
 
-    if (!cls.is(Module) && cls.classParents.exists(_.classSymbol eq defn.PhantomClass))
-      ctx.error(PhantomIsInObject(), cdef.pos)
+    if (cls.classParents.exists(_.classSymbol eq defn.PhantomClass)) {
+      if (!cls.is(Module))
+        ctx.error(PhantomIsInObject(), cdef.pos)
+      else if (!cls.owner.is(Module) && !cls.owner.is(Package))
+        ctx.error(PhantomObjectIsInPackageOrObject(), cdef.pos)
+    }
 
     // check value class constraints
     checkDerivedValueClass(cls, body1)

tests/neg/phantom-trait-4.scala

@@ -0,0 +1,23 @@
+
+class Foo {
+
+  object Boo1 extends Phantom // error
+
+  def foo = {
+    object Boo2 extends Phantom // error
+    42
+  }
+}
+
+object Foo {
+  object Boo1 extends Phantom
+
+  def foo = {
+    object Boo2 extends Phantom // error
+    42
+  }
+}
+
+package foo {
+  object Boo1 extends Phantom
+}