Narrowing

[kaleidawave]

Using Type::Constructor information in a condition, it should be possible to record some truths that follow in the current block. Then getting-information-based-thingies should be able to read these recorded information and change results based from it.

This will use similar structs to the ones added (eventually) in #35

[kaleidawave]

Work started in #156

There should be a new type Type::Narrowed { /* points to a poly type */ on: TypeId, narrowed: TypeId }. This retains some information through on but contains narrowed which replaces the base type (get_constraint should be changed to look this up).

[kaleidawave]

Note that there are some problems when narrowing properties. In the case that the property is a getter the following has a type error that is not caught by TSC.

function func(param: { a: number | string }) {
	if (typeof param.a === "number") {
		return Math.sin(param.a)
	}
}

func({ get param() { return Date.now() % 2 ? 5 : "string "} })

Half the time this will fail (produce undesired results in this example) at runtime.

Some thoughts

• Don't narrow properties
• Maybe a special type for disallowing getters or impure getters
• Track narrowing via events