Differing user-defined type guard and 'typeof' type guard behaviour when narrowing 'any'

In the below code, using a typeof type guard and an equivalent (I thought) user-defined guard, only one error is produced.

var y: any;

// Built-in type guard
if (typeof y === "string") {
    y.hello = true; // Correct error - 'hello' does not exist on type string
}

// Equivalent user-defined type guard
function f(x: any): x is string {
    return typeof x === "string";
}

if (f(y)) {
    y.hello = true; // No error with user-defined guard
}

Playground demo. Looks like user-defined type guards won't narrow from any, in any circumstances, as far as I can tell.

从 issue 中的 TypeScript Playground 示例开始，重现内置 guard 和用户定义 guard 产生的不同诊断。跟踪编译器对 any 值的 narrowing 行为，并验证用户定义的 guard 对 y.hello 产生的错误与 typeof guard 相同。