Scala 2 overrides a return type to an inferred type; Scala 3 does not

[SethTisue]

scala 2.13.5> class C; class D extends C
scala 2.13.5> class Test { def foo: C = new C }
scala 2.13.5> class Test2 extends Test { override def foo = new D }
scala 2.13.5> (new Test2).foo
val res0: D = D@5d32c1c7

whereas in Scala 3 the type of res0 is C

I'm fairly sure this is intended behavior in Scala 3, but I can't seem to find either a ticket, or a migration guide entry about it.

There is a ticket complaining about the Scala 2 behavior: scala/bug#7212

[adpi2]

Thanks for reporting!

Do you think this pattern could cause a compiler error when porting a valid piece of Scala 2.13 code to Scala 3?

[SethTisue]

Yes, it happened here: scala/scala-parser-combinators#370

[dwijnand]

scala/scala-parser-combinators#370 isn't a compiler error, it's a runtime error. And I still don't really think either compiler's at fault, as it's just some crazy class casting code

      def apply(in: Input) = {
        /*
         * transformed reader
         */
        val inMem = in.asInstanceOf[PackratReader[Elem]]

[SethTisue]

isn't a compiler error, it's a runtime error

fair. in any case, it's a potential migration concern.