Type inference of Array receiver with extension method

[som-snytt]

Minimized code

Starting scala3 REPL...
scala> import scala.util.chaining._

scala> val array = new Array[AnyRef](8).tap(xs => xs(7) = "hi")
val array: Array[AnyRef] = Array(null, null, null, null, null, null, null, hi)

scala> new Array(16)
val res0: Array[Nothing] = Array(null, null, null, null, null, null, null, null, null, null, null, null, null, null, null, null)

scala> res0.getClass.getComponentType
val res1: Class[?] = class scala.runtime.Nothing$

scala> new Array(16): Array[AnyRef]
val res2: Array[AnyRef] = Array(null, null, null, null, null, null, null, null, null, null, null, null, null, null, null, null)

scala> res2.getClass.getComponentType
val res3: Class[?] = class java.lang.Object

scala> new Array(16).tap(Array.copy(array, 0, _, 0, 8)) : Array[AnyRef]
1 |new Array(16).tap(Array.copy(array, 0, _, 0, 8)) : Array[AnyRef]
  |^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |Found:    Array[Nothing]
  |Required: Array[AnyRef]

scala> new Array(16).tap(Array.copy(array, 0, _, 0, 8))
java.lang.ArrayStoreException: java.lang.String
        at scala.runtime.ScalaRunTime$.array_update(ScalaRunTime.scala:75)

Output in Scala 2

scala 2.13.3> import scala.util.chaining._
import scala.util.chaining._

scala 2.13.3> val array = new Array[AnyRef](8).tap(xs => xs(7) = "hi")
val array: Array[AnyRef] = Array(null, null, null, null, null, null, null, hi)

scala 2.13.3> new Array(16).tap(Array.copy(array, 0, _, 0, 8))
val res0: Array[Nothing] = Array(null, null, null, null, null, null, null, hi, null, null, null, null, null, null, null, null)

scala 2.13.3> new Array(16).tap(Array.copy(array, 0, _, 0, 8)): Array[AnyRef]
val res1: Array[AnyRef] = Array(null, null, null, null, null, null, null, hi, null, null, null, null, null, null, null, null)

scala 2.13.3> new Array(16).getClass.getComponentType
val res3: Class[_] = class java.lang.Object

scala 2.13.3> new Array(16).tap(Array.copy(array, 0, _, 0, 8)): Array[AnyRef]  // print
((scala.util.`package`.chaining.scalaUtilChainingOps[Array[AnyRef]](scala.reflect.`package`.materializeClassTag[AnyRef]().newArray(16)).tap[Unit](((x$1: Array[AnyRef]) => scala.Array.copy(array, 0, x$1, 0, 8)))): scala.Array[scala.AnyRef]) // : Array[AnyRef]

Expectation

Array type inferred as in Scala 2.

Not sure about the component type of Array[Nothing]. Best of all would be to ensure statically that the array is of length zero.

There are related issues, but this is "nothing fancy," just "plumbing." Fancy[Nothing].

Noticed at scala/scala#9275 (comment)

[odersky]

I think that dotty's behavior is reasonable. There's some funky behavior with Nothing avoidance in Scala 2 that is not repeated. It seems this kicks in here. Is there are reasonable code sample where this matters?

[som-snytt]

Two changes from Scala 2 are

1. new Array(16) is Array[Nothing] but in Scala 2 you actually get an Array[Object] instead of Array[Nothing$]
2. new Array(16).tap(initialize): Array[AnyRef] infers new Array[AnyRef] in Scala 2 but new Array[Nothing] in Scala 3

The second expression fails because of tap. That seems like a limitation. There are related tickets about type inference and extension methods which might subsume this one.

The first difference might be a Scala 3 feature.

[smarter]

We should just completely forbid Array[Nothing] and force people to write down a type parameter, we know it's unsound and already forbid inferring a ClassTag[Nothing] for that reason: #1730

[som-snytt]

I agree on forbidding Array[Nothing] (though there are weird APIs that take an array just for the type, and someone will go, What do you mean I can't have an empty array of nothing?).

It would be nice to infer Array[Something] when that is the expected type.

[kynthus]

I don't know if it's related, but the following 2D array initialization seems to fail.

Starting scala3 REPL...
Scala compiler version 3.0.0-M3 -- Copyright 2002-2020, LAMP/EPFL
scala> val c: Array[Array[Int]] = Array(new Array(5))
1 |val c: Array[Array[Int]] = Array(new Array(5))
  |                                 ^^^^^^^^^^^^
  |                                 Found:    Array[Nothing]
  |                                 Required: Array[Int]

Can be inferred in Scala 2.13.4.

Welcome to Scala 2.13.4 (Java HotSpot(TM) 64-Bit Server VM, Java 1.8.0_202).
Type in expressions for evaluation. Or try :help.

scala> val c: Array[Array[Int]] = Array(new Array(5))
val c: Array[Array[Int]] = Array(Array(0, 0, 0, 0, 0))