Port "illegal cyclic type reference" error to new scheme

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

@@ -147,7 +147,8 @@ enum ErrorMessageID extends java.lang.Enum[ErrorMessageID] {
     StaticFieldsShouldPrecedeNonStaticID,
     IllegalSuperAccessorID,
     TraitParameterUsedAsParentPrefixID,
-    UnknownNamedEnclosingClassOrObjectID
+    UnknownNamedEnclosingClassOrObjectID,
+    IllegalCyclicTypeReferenceID
 
   def errorNumber = ordinal - 2
 }

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

@@ -2392,4 +2392,11 @@ object messages {
       |current scope.
       """.stripMargin
     }
+
+    case class IllegalCyclicTypeReference(sym: Symbol, where: String, lastChecked: Type)(implicit val ctx: Context)
+      extends Message(IllegalCyclicTypeReferenceID) {
+      val kind: String = "Type"
+      val msg: String = i"illegal cyclic type reference: ${where} ${hl(lastChecked.show)} of $sym refers back to the type itself"
+      val explanation: String = ""
+    }
 }

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

@@ -300,7 +300,7 @@ object Checking {
     catch {
       case ex: CyclicReference =>
         if (reportErrors)
-          errorType(i"illegal cyclic reference: ${checker.where} ${checker.lastChecked} of $sym refers back to the type itself", sym.sourcePos)
+          errorType(IllegalCyclicTypeReference(sym, checker.where, checker.lastChecked), sym.sourcePos)
         else info
     }
   }

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

@@ -1672,4 +1672,19 @@ class ErrorMessagesTests extends ErrorMessagesTest {
       val UnknownNamedEnclosingClassOrObject(name) :: Nil = messages
       assertEquals("doesNotExist", name.show)
     }
+
+  @Test def illegalCyclicTypeReference() =
+    checkMessagesAfter(RefChecks.name) {
+      """
+        |type X = List[X]
+      """.stripMargin
+    }
+    .expect { (ictx, messages) =>
+      implicit val ctx: Context = ictx
+      assertMessageCount(1, messages)
+      val IllegalCyclicTypeReference(sym, where, lastChecked) :: Nil = messages
+      assertEquals("type X", sym.show)
+      assertEquals("alias", where)
+      assertEquals("List[X]", lastChecked.show)
+    }
 }