Better union support

[ScottPeterJohnson]

Currently type unions in return types get converted to "dynamic". This is pretty unsafe. I'm proposing a safer method, with some Kotlin code attached.
Given a function that returns string|number, generate:

external fun someFunction() : StringOrNumber;
external class StringOrNumber
inline fun <T> StringOrNumber.destructure(
		onString : (String)->T,
		onNumber: (Number)->T
) : T{
	if("string".equals(jsTypeOf(this))){
		return onString(this.unsafeCast<String>())
	} else if("number".equals(jsTypeOf(this))){
		return onNumber(this.unsafeCast<Number>())
	} else {
		throw IllegalStateException("Value is neither type 'string' nor type 'number': ${this}")
	}
}

The inline function would have to be generated in a separate Kotlin file, but it could make for safer code, which might improve the quality of the converter itself. We'd need a type discriminant per type, however. Some libraries define their own type discriminants, but following a basic primitive type/known disjoint properties method could be helpful, especially with string literal types.
I can try to generate a proof of concept implementation if this seems okay.

[bashor]

Sorry for the delay!
It looks too verbose to me. One possible solution that we discussed is using an annotation on the type and write some checker in Kotlin JS frontend.

[ScottPeterJohnson]

Oh, that solution actually does sound better. I'll close this issue in favor of that.

[bashor]

Anyway, thank you for the request