Control flow with type guards forget previously computed props

[timocov]

TypeScript Version: 2.5.3 / nightly (2.6.0-dev.20171005)

Code

interface Base {
	value?: string;
}

interface A extends Base {
	type: 'A';
	a: number;
}

interface B extends Base {
	type: 'B';
	b: string;
}

type Common = A | B;

declare var a: Common;

if (a.value !== undefined && a.type === 'A') {
	const str: string = a.value; // Error
}

if (a.type === 'A' && a.value !== undefined) {
	const str: string = a.value; // Ok
}

Expected behavior:
No errors.

Actual behavior:

error TS2322: Type 'string | undefined' is not assignable to type 'string'.
  Type 'undefined' is not assignable to type 'string'.

[mhegazy]

The first check narrows type of a.value to string, but the second check then narrows the type of a to A. Need to think of possible solutions here. It is possible to address this by fixing issues like #18840.