Companion module should be deprecated if class is deprecated

[som-snytt]

Compiler version

3.0.0

Minimized code

@deprecated("will be removed", "0.1")
abstract class J
object J {
  def f(i: Int): String = i + "th"
}

Output

➜  snips scalac -deprecation -d /tmp -source 3.0-migration kdep.scala
-- Deprecation Warning: kdep.scala:4:28 ------------------------------------------------------------------------------
4 |  def f(i: Int): String = i + "th"
  |                          ^^^
  |method + in class Int is deprecated since 2.13.0: Adding a number and a String is deprecated. Use the string interpolation `s"$num$str"`
1 warning found

Expectation

In Scala 2, companion is deprecated if class is. No warning is emitted.

That was for scala/bug#2799 which is currently #11022.

Extending a deprecated class is not an exemption:

@deprecated("will be removed", "0.1")
abstract class C {
  def f(i: Int): String
}
// warn 2x
class D extends C {
  override def f(i: Int): String = i + "th"
}

@deprecated("will be removed", "0.1")
abstract class K {
  def f(i: Int): String
}
// nowarn in Scala 2 by companion rule, e.g., `scala.io.Position`
object K extends K {
  override def f(i: Int): String = i + "th"
}

Arguably, extending a deprecated class should also suppress deprecations. If I'm not supposed to extend, then it should be marked @deprecatedInheritance. (It's not outlandish that a base class is deprecated, for removal, but the subclass is not.)

[sjrd]

In Scala 2, companion is deprecated if class is. No warning is emitted.

I think Scala 2 is at fault here. If the class is deprecated, the companion is not deprecated. It's only the warnings that get erroneously silenced. Actually that allows to create holes in the deprecation chain:

>scala -deprecation
Welcome to Scala 2.13.4 (OpenJDK 64-Bit Server VM, Java 1.8.0_222).
Type in expressions for evaluation. Or try :help.

scala> @deprecated("class is deprecated", "1.0.0") class Foo; object Foo { def bar: Int = 42 }
class Foo
object Foo

scala> new Foo()
           ^
       warning: class Foo is deprecated (since 1.0.0): class is deprecated
val res0: Foo = Foo@70c491b8

scala> Foo.bar
val res1: Int = 42

scala> @deprecated("class is deprecated", "1.0.0") class Foobar; object Foobar { def bar: Int = { new Foo(); 42 } }
class Foobar
object Foobar

scala> Foobar.bar
val res2: Int = 42

Observe that:

• new Foo() triggers a deprecation warning
• Foo.bar doesn't (so no, the companion itself is not deprecated)
• The new Foo() inside Foobar.bar does not emit a warning, although it should
• Through Foobar.bar, which does not emit a deprecation warning, I can call new Foo(), which was initially deprecated

So Scala 3 does the right thing here. Scala 2 is to blame.

[odersky]

Should we close this then?

[som-snytt]

I agree the behavior is dubious.