Type guards fail with external typeof check

[jednano]

Related to #2388

TypeScript Version: 2.1.5 (Playground)

Code

function foo(x: number | string) {
    if (isString(x)) {
        x.substr(1); // <-- Error
    }
}

function isString(x: any) {
    return typeof x === 'string';
}

Expected behavior:

No errors.

Actual behavior:

Error: Property 'substr' does not exist on type 'string | number'.
       Property 'substr' does not exist on type 'number'.

Value Proposition:

This is a common thing to do with external libraries (e.g., Lodash):

//_.isString
interface LoDashStatic {
    /**
     * Checks if value is classified as a String primitive or object.
     *
     * @param value The value to check.
     * @return Returns true if value is correctly classified, else false.
     */
    isString(value?: any): value is string;
}

https://github.com/DefinitelyTyped/DefinitelyTyped/blob/master/lodash/index.d.ts#L12586-L12595
https://lodash.com/docs/4.17.4#isString

[blakeembrey]

It works fine for me, you just didn't annotate the function:

function foo(x: number | string) {
    if (isString(x)) {
        x.substr(1); // <-- Error
    }
}

function isString(x: any): x is string {
    return typeof x === 'string';
}

Or is the issue about automatically inferring type guards based on the function? In that case, I imagine it's a little more complicated.

[jednano]

Thanks @blakeembrey!