Feature Request: Infer types when using variable assignment with if-statements.

[ccorcos]

TypeScript Version: 2.6.0-dev.201xxxxx

Code

function f(x: string | undefined) {
	if (x) {
		const l = x.length
	}
	const y = !!x
	if (y) {
                // type error with strictNullChecks on.
		const l = x.length
	}
	if (!!x) {
		const l = x.length
	}
}

Expected behavior:

I would expect this code to compile without errors by simply refactoring some conditions into variables.

Actual behavior:

Unless the entire condition lives inside the if statement, there will be a type error.

[ahejlsberg]

This is a design limitation. The control flow analyzer doesn't track what effects a particular value for one variable implies for other variables. See #12184 for a similar example.