Type mismatch with a + operator in an extension method of a newtype

[hmemcpy]

Scala version: 2.13.13 (under Xsource:3)

The following code, under Xsource:3 with Scala 2.13.13:

  type Point = Point.Type
  object Point {
    type Type <: Any
    def apply(x: Int): Point = x.asInstanceOf[Point]

    implicit final class MyExt(val myInt: Point) extends AnyVal {
      def +(other: Int): Point = ???
    }
  }

  val p = Point(3)
  val p2 = p + 2

Produces the following error:

type mismatch;
 found   : Int(2)
 required: String
    val p2 = p + 2

Problem

The problem started after upgrading to Scala 2.13.13. We use the scala-newtype library, and some of our newtypes that define a + operator started to fail with the error above.
The code snippet is the simplified "expanded" version of the newtype macro that also exhibits the problem:

@newtype case class Point(i: Int)
  def +(other: Int): Point
}

In all previous versions, this code compiled fine under Xsource:3.

[SethTisue]

As of 2.13.13, -Xsource:3 only causes warnings to be omitted; it never (unless we missed something!) causes other behavior changes.

So it seems expected to me that if the code used to require -Xsource:3 to compile, it might now instead require -Xsource:3-cross, which does enable behavior changes.

[SethTisue]

By the way, we have done our best to be clear about this in the 2.13.13 release notes as well as the PR description at scala/scala#10573 — if you think further edits would help make the situation clearer, let us know.

[som-snytt]

I confirmed that it needs -Xsource:3-cross.

[hmemcpy]

Yeah, was about to update that it does work under 3-cross. Thanks for checking!

[som-snytt]

import is not necessary under -Xsource:3-cross.

https://github.com/scala/scala/blob/2.13.x/src/compiler/scala/tools/nsc/typechecker/Contexts.scala#L1078

[hmemcpy]

Right, sorry, I deleted my comment. We can't switch to 3-cross (yet) because of the overridden member type inference change - it causes a lot of type mismatches for us (because we omit type signatures practically everywhere) and it doesn't play nice with ZIO's contravariant R parameter (something I encountered here #12707). Unfortunately, fixing it on our side is too "expensive" in terms of changes...

Anyway, with the Xsource:3 changes I am finally able to upgrade our codebase to the latest Scala 2.13.13 without (many) additional changes, except this one (and another one having to do with infix operators beginning on a new line).

Anyway, thanks for checking, let's keep this issue closed.

[som-snytt]

We didn't mention that -Yno-predef also suffices for the example; or -Yimports to provide a custom Predef that omits what is undesirable and supplies what is necessary.

[lrytz]

because we omit type signatures practically everywhere

Is there a technical reason or would everything work with explicit types?

fixing it on our side is too "expensive" in terms of changes

With 2.13.13 you could try -quickfix:msg=under -Xsource:3-cross, the inferred type changes to have the compiler add the result types