Retain type guard info

[electricessence]

Is a bit of a low value feature, but TS is getting smarter all the time so why not:

If you can assert that a value is the result of a type guard. Why not keep that information for later?

function report(unknown:Object) {
	const t = typeof unknown;
	console.log(unknown, "is a",t);
	switch(t) // ** 
	{
		case "string":
			let s:string = unknown; // ** should compile without warning.
			// do string operations with s.
			break;

		// More similar cases...
	}
}

[normalser]

#12184 (comment)