Check safe initialization of global objects

[liufengyun]

Check safe initialization of global objects

Fix #10549, #9176

Example 1

Given the code below:

object A {
  val a: Int = B.b     // error
}

object B {
  val b: Int = A.a     // error
}

The checker produces the following warning:

-- Warning: tests/init/full/neg/global-cycle1.scala:2:6 ------------------------
2 |  val a: Int = B.b     // error
  |      ^
  |      Access non-initialized value a. Calling trace:
  |       -> val a: Int = B.b     // error	[ global-cycle1.scala:2 ]
  |        -> val b: Int = A.a     // error	[ global-cycle1.scala:6 ]
-- Warning: tests/init/full/neg/global-cycle1.scala:6:6 ------------------------
6 |  val b: Int = A.a     // error
  |      ^
  |      Access non-initialized value b. Calling trace:
  |       -> val b: Int = A.a     // error	[ global-cycle1.scala:6 ]
  |        -> val a: Int = B.b     // error	[ global-cycle1.scala:2 ]

Example 2

Given

class Foo(val opposite: Foo)
case object A extends Foo(B) // error
case object B extends Foo(A) // error

The checker reports:

-- Warning: tests/init/full/neg/i9176.scala:2:0 --------------------------------
2 |case object A extends Foo(B) // error
  |^
  |Access non-initialized object A. Calling trace:
  | -> case object A extends Foo(B) // error	[ i9176.scala:2 ]
  |  -> case object B extends Foo(A) // error	[ i9176.scala:3 ]
-- Warning: tests/init/full/neg/i9176.scala:3:0 --------------------------------
3 |case object B extends Foo(A) // error
  |^
  |Access non-initialized object B. Calling trace:
  | -> case object B extends Foo(A) // error	[ i9176.scala:3 ]
  |  -> case object A extends Foo(B) // error	[ i9176.scala:2 ]

[liufengyun]

Superseded by #11385, #11913, #11278 and #11533