Improve online Kotlin documentation for the `where` clause

1

@gidds brought to my attention that the where keyword makes an intersection type, not a union type. I assumed the opposite. The docs should be more explicit.

Under the online documentation for Generics: Constraints, it says, “Only one upper bound can be specified inside the angle brackets. If the same type parameter needs more than one upper bound, we need a separate where -clause”

I recommend adding a little more, like, “A where clause creates an intersection type - each sub-clause must be satisfied. The type parameter specified this way must be a subtype of A and B and C etc.”

This little code snippet proves the above:

// Create some interfaces...
interface A {
    val foo: String
}
interface B {
    val bar: Int
}

// Type T must be both and A and a B
fun <T> baz(t: T): String where T : A, T : B {
    return t.foo + t.bar
}

// Now test baz()
fun fud(): String {

    // ERROR: Inferred type is not a subtype of B
//    val t = object: A {
//        override val foo: String = "hi"
//    }

    // ERROR: Inferred type is not a subtype of A
//    val t = object: B {
//        override val bar: Int = 5
//    }

    // Works - t must be a subtype of both A and B.
    val t = object: A, B {
        override val foo: String = "hi"
        override val bar: Int = 5
    }
    return baz(t) // returns "hi5"
}
2

Why not simply fork the documentation and submit a PR?
https://github.com/JetBrains/kotlin-web-site/edit/master/pages/docs/reference/generics.md

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3

I did it! I read that you need a docker container to build the docs and imagined a long process. I was unsure if what I had to say was correct and worthy of being in the docs.

I never did the tiny fork/pull-request turnaround through the GitHub UI before. It was really easy! I guess I just needed a little encouragement. Thank you for that!

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4

Hi!
Thanks for the addition.
I’ve suggested minor formatting and wording changes in the PR. We wrap names of functions and classes in backticks.

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