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Why does the "contain" method in "Option" use the second type with a lower border instead of the "Any" type?

I was looking at the Option class in Scala and came across a contains method:

final def contains[A1 >: A](elem: A1): Boolean

The option is covariant, so I understand that it cannot just use A as an elem type. However, given that type A1 is never used, why the method cannot be like this:

 final def contains(elem: Any): Boolean

Is it just a style or am I missing something important?

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2 answers

No reason, it's just an oversight. In fact, this is not even the only one. Either.joinRight , for example, also contains unnecessary lower bounds. But since their removal will mean a break in the initial and binary compatibility, this has not yet happened.

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This is directly related to the fact that Option covariant and is actually required in order for the types to match.

 class Foo(x: Int) object Bar entends Foo(0) def foo(t: Option[Foo]) = t contains (new Foo(2))

I can actually pass in the type Option[Bar.type] , and it will "work" because Bar is Foo , and the type of type constraint requires that it also accept types that are supertypes of parameterized type A

 val res = foo(Some(Bar)) >> res0: Boolean = false

Note that the above actually calls contains in the instance of a Option[Bar.type] . Therefore, [A1 >: A] .

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