Inaccurate warning with -Ysafe-init with enum and nested object in companion

[TVerse]

Compiler version

Scala 3.0.0 with compiler flag -Ysafe-init.

Minimized code

enum Enum:
  case Case

object Enum:
  object nested:
    val a: Enum = Case

  val b: Enum = f(nested.a)

  def f(e: Enum): Enum = e

@main def main(): Unit = println(Enum.b)

Output

> sbt run
<SNIP>
[warn] -- Warning: <PATH>
[warn] 8 |  val b: Enum = f(nested.a)
[warn]   |                  ^^^^^^^^
[warn]   |Promoting the value under initialization to fully-initialized. Calling trace:
[warn]   | -> val b: Enum = f(nested.a) [ Enum.scala:8 ]
[warn] one warning found
[warn] one warning found
[info] running main
Case

Expectation

No warning. Given that it prints correctly, the value was correctly initialized. I believe there's no real initialization order issue here: when b initializes, nested gets initialized and nested.a with it.

Things that do not cause a warning:

• Using a case class, Int, or String instead of the enum
• Renaming the outer object so it's not a companion anymore
• val a: Enum = ??? (this breaks when printing of course)

[liufengyun]

Thanks for your report @TVerse . It's fixed in the master, and will be part of 3.0.1.

[liufengyun]

The promotion warning happens when we pass an instance of inner class as method arguments.

For example, given the following program:

def g(b: O.B): Int = b.foo()

object O:
  class B() { def foo() = n + 1 }
  g(new B())
  val n: Int = 10

The checker reports the following warning:

-- Warning: tests/init/neg/i12544.scala:5:4 ------------------------------------
5 |  g(new B())
  |    ^^^^^^^
  |    Promoting the value to fully-initialized is unsafe.
  |     Calling trace:
  |     -> g(new B())	[ i12544.scala:5 ]
  |
  |    The unsafe promotion may cause the following problem(s):
  |
  |    1. Access non-initialized value n. Calling trace:
  |     -> g(new B())	[ i12544.scala:5 ]
  |       -> class B() { def foo() = n + 1 }	[ i12544.scala:4 ]

[TVerse]

Thank you!