StackOverflow generated by Pattern Matching

[hamzaremmal]

Compiler version

3.7.2

Minimized code

object Foo {
  override def equals(that : Any) = that match {
    case _: this.type => true
    case _            => false
  }
}

val _ = Foo.equals(Foo)

Calling Foo.equals(Foo) will generate a Stack Overflow at Runtime

Output

java.lang.StackOverflowError
	at Playground$Foo$.equals(main.scala:5)
	at Playground$Foo$.equals(main.scala:5)
	at Playground$Foo$.equals(main.scala:5)
	at Playground$Foo$.equals(main.scala:5)
	at Playground$Foo$.equals(main.scala:5)
	at Playground$Foo$.equals(main.scala:5)
	at Playground$Foo$.equals(main.scala:5)
	at Playground$Foo$.equals(main.scala:5)
	at Playground$Foo$.equals(main.scala:5)
	at Playground$Foo$.equals(main.scala:5)
	at Playground$Foo$.equals(main.scala:5)
	at Playground$Foo$.equals(main.scala:5)
	at Playground$Foo$.equals(main.scala:5)
	at Playground$Foo$.equals(main.scala:5)
	at Playground$Foo$.equals(main.scala:5)
	at Playground$Foo$.equals(main.scala:5)
	at Playground$Foo$.equals(main.scala:5)
	at Playground$Foo$.equals(main.scala:5)
	at Playground$Foo$.equals(main.scala:5)
	at Playground$Foo$.equals(main.scala:5)
	at Playground$Foo$.equals(main.scala:5)
	at Playground$Foo$.equals(main.scala:5)
	at Playground$Foo$.equals(main.scala:5)
	at Playground$Foo$.equals(main.scala:5)
	at Playground$Foo$.equals(main.scala:5)
	at Playground$Foo$.equals(main.scala:5)

Expectation

To compile and run as expected

Notes

A similar pattern is used by the stdlib in:

scala3/library/src/scala/collection/immutable/IntMap.scala

Lines 61 to 71 in 48bc891

	private[immutable] case object Nil extends IntMap[Nothing] {
	// Important! Without this equals method in place, an infinite
	// loop from Map.equals => size => pattern-match-on-Nil => equals
	// develops. Case objects and custom equality don't mix without
	// careful handling.
	override def equals(that : Any) = that match {
	case _: this.type => true
	case _: IntMap[_] => false // The only empty IntMaps are eq Nil
	case _ => super.equals(that)
	}
	}