Add an error message class for phantom types

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

@@ -105,6 +105,7 @@ public enum ErrorMessageID {
     ExpectedTypeBoundOrEqualsID,
     ClassAndCompanionNameClashID,
     TailrecNotApplicableID,
+    OnlyStaticObjectsCanExtendPhantomID,
     ;
 
     public int errorNumber() {

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

@@ -1826,4 +1826,22 @@ object messages {
       val explanation =
        hl"A method annotated ${"@tailrec"} must be declared ${"private"} or ${"final"} so it can't be overridden."
   }
+
+  case class OnlyStaticObjectsCanExtendPhantom(symbol: Symbol)(implicit ctx: Context)
+    extends Message(OnlyStaticObjectsCanExtendPhantomID) {
+    val kind = "Syntax"
+    val msg = "only static objects can extend scala.Phantom"
+    val explanation = {
+      val codeExample = "object MyPhantoms extends Phantom"
+
+      hl"""
+        |Phantom types are definded by an object extending scala.Phantom.
+        |This object will represent a universe of phantom types that is completely separated from types in scala.Any or other phantom universes.
+        |We can define our phantom universe MyPhantoms:
+        |
+        |$codeExample
+        |
+      """.stripMargin
+    }
+  }
 }

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

@@ -1427,7 +1427,7 @@ class Typer extends Namer with TypeAssigner with Applications with Implicits wit
 
       // Check that phantom lattices are defined in a static object
       if (cls.classParents.exists(_.typeSymbol eq defn.PhantomClass) && !cls.isStaticOwner)
-        ctx.error("only static objects can extend scala.Phantom", cdef.pos)
+        ctx.error(OnlyStaticObjectsCanExtendPhantom(cls), cdef.pos)
 
       // check value class constraints
       checkDerivedValueClass(cls, body1)

compiler/test/dotty/tools/dotc/reporting/ErrorMessagesTests.scala

@@ -1051,4 +1051,17 @@ class ErrorMessagesTests extends ErrorMessagesTest {
       assertEquals("trait G", other.show)
 
     }
+
+  @Test def onlyStaticObjectsCanExtendPhantom =
+    checkMessagesAfter("frontend") {
+      "class T extends Phantom"
+    }.expect { (ictx, messages) =>
+      implicit val ctx: Context = ictx
+
+      assertMessageCount(1, messages)
+
+      val OnlyStaticObjectsCanExtendPhantom(cls) :: Nil = messages
+
+      assertEquals("class T", cls.show)
+    }
 }