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kotlinlang

is it idiomatic to use `T?` like an optional of type `T`?

Yes, though I guess it depends on what you need to express. `T?` is meant to express "the nullable version of T, no matter whether T is already nullable or not". Note that an unconstrained T is already potentially nullable, depending on the type parameter that's actually passed in a specific usage of your generic declaration

As Joffrey Bion mention you'd have to constraint your type `T`  to `T: Any`  then you should be able to use `T?`  as an "optional" `T`

as in, `val foo: Foo?` is nullable, but `val foo: Foo` is not, correct?

No for concrete types it's not same, the implicit nullability of the type is only true for generics

`Foo` is not nullable, `Foo?` is nullable. Both can be passed as `T` to a generic declaration if `T` is not constrained

oh, yes. that also aligns with my understanding. thanks about that clarification.

so, just making sure: say I’m making a Kotlin class, and I need an optional field (of a non-generic type, let’s say `Foo`), I should use a `Foo?`, correct?