Add error message when trait parameter prefixes are used for its parent

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

@@ -145,7 +145,8 @@ enum ErrorMessageID extends java.lang.Enum[ErrorMessageID] {
     NoMatchingOverloadID,
     StableIdentPatternID,
     StaticFieldsShouldPrecedeNonStaticID,
-    IllegalSuperAccessorID
+    IllegalSuperAccessorID,
+    TraitParameterUsedAsParentPrefixID
 
   def errorNumber = ordinal - 2
 }

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

@@ -2362,4 +2362,20 @@ object messages {
     }
     val explanation: String = ""
   }
+
+  case class TraitParameterUsedAsParentPrefix(cls: Symbol)(implicit val ctx: Context)
+    extends Message(TraitParameterUsedAsParentPrefixID) {
+    val kind: String = "Reference"
+    val msg: String =
+      s"${cls.show} cannot extend from a parent that is derived via its own parameters"
+    val explanation: String =
+      ex"""
+          |The parent class/trait that ${cls.show} extends from is obtained from
+          |the parameter of ${cls.show}. This is disallowed in order to prevent
+          |outer-related Null Pointer Exceptions in Scala.
+          |
+          |In order to fix this issue consider directly extending from the parent rather
+          |than obtaining it from the parameters of ${cls.show}.
+          |""".stripMargin
+  }
 }

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

@@ -116,7 +116,7 @@ object RefChecks {
       case TypeRef(ref: TermRef, _) =>
         val paramRefs = ref.namedPartsWith(ntp => ntp.symbol.enclosingClass == cls)
         if (paramRefs.nonEmpty)
-          ctx.error("trait parameters cannot be used as parent prefixes", parent.sourcePos)
+          ctx.error(TraitParameterUsedAsParentPrefix(cls), parent.sourcePos)
       case _ =>
     }
 

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

@@ -1638,4 +1638,23 @@ class ErrorMessagesTests extends ErrorMessagesTest {
         message.msg
       )
     }
+
+  @Test def traitParametersUsedAsParentPrefix() =
+    checkMessagesAfter(RefChecks.name) {
+      """
+        |class Outer {
+        |   trait Inner
+        |   trait Test(val outer: Outer) extends outer.Inner
+        |}
+        |""".stripMargin
+    }.expect {
+      (ictx, messages) =>
+        implicit val ctx: Context = ictx
+        val TraitParameterUsedAsParentPrefix(cls) :: Nil = messages
+        assertEquals("trait Test", cls.show)
+        assertEquals(
+          s"${cls.show} cannot extend from a parent that is derived via its own parameters",
+          messages.head.msg
+        )
+    }
 }