-Yexplicit-nulls exhibits false negative when combining try/match

[theosotr]

Compiler version

3.6.0-RC1-bin-SNAPSHOT-git-e9d4831

Minimized code

Related to #21380

@main def test(): Unit = {
  var x: String | Null = null
  x = ""
  var i: Int = 1
  try {
    i match {
      case _ => {
        x = null
        throw new Exception()
      }
    }
    x = ""
  }
  catch {
    case (e: Exception) => {}
  }
  x.replace("", "")
}

Output

Compiling the program with -Yexplicit-nulls results in the following error at runtime

java.lang.NullPointerException: Cannot invoke "String.replace(java.lang.CharSequence, java.lang.CharSequence)" because "x" is null
	at test$package$.test(test.scala:17)
	at test.main(test.scala:1)
	at java.base/jdk.internal.reflect.DirectMethodHandleAccessor.invoke(DirectMethodHandleAccessor.java:103)
	at java.base/java.lang.reflect.Method.invoke(Method.java:580)
	at dotty.tools.runner.RichClassLoader$.run$extension$$anonfun$1(ScalaClassLoader.scala:36)
	at dotty.tools.runner.ScalaClassLoader$.asContext(ScalaClassLoader.scala:80)
	at dotty.tools.runner.RichClassLoader$.dotty$tools$runner$RichClassLoader$$$asContext$extension(ScalaClassLoader.scala:18)
	at dotty.tools.runner.RichClassLoader$.run$extension(ScalaClassLoader.scala:36)
	at dotty.tools.runner.CommonRunner.run(ObjectRunner.scala:23)
	at dotty.tools.runner.CommonRunner.run$(ObjectRunner.scala:13)
	at dotty.tools.runner.ObjectRunner$.run(ObjectRunner.scala:48)
	at dotty.tools.runner.CommonRunner.runAndCatch(ObjectRunner.scala:30)
	at dotty.tools.runner.CommonRunner.runAndCatch$(ObjectRunner.scala:13)
	at dotty.tools.runner.ObjectRunner$.runAndCatch(ObjectRunner.scala:48)
	at dotty.tools.MainGenericRunner$.run$1(MainGenericRunner.scala:215)
	at dotty.tools.MainGenericRunner$.process(MainGenericRunner.scala:270)
	at dotty.tools.MainGenericRunner$.main(MainGenericRunner.scala:281)
	at dotty.tools.MainGenericRunner.main(MainGenericRunner.scala)

Expectation

The program should have been rejected.

[noti0na1]

We should make NotNullInfo remembering the last info, and provide a way to collect refs ever being retracted.

cc @olhotak