Strange stateful behavior with inferred type predicate type

[ghost]

TypeScript Version: nightly (2.5.0-dev.20170725)

Code

function tryCast<T, U extends T>(value: T, predicate: (t: T) => t is U): U | undefined {
    return predicate(value) ? value : undefined;
}

declare const sn: string | number;

const x: number | undefined = tryCast(sn, (n): n is number => typeof n === "number"); // Always works

const y = tryCast(sn, (n): n is number => typeof n === "number");

const z: number | undefined = y;

Expected behavior:

No error.

Actual behavior:

src/a.ts(11,7): error TS2322: Type 'string | number | undefined' is not assignable to type 'number | undefined'.
  Type 'string' is not assignable to type 'number | undefined'.

Depending on the order I hover over things, hovering over tryCast and y might show a type of string | number | undefined, but it might show a type of number | undefined. (But the error at z is always present.)