union types not type guarded

[npenin]

TypeScript Version: 3.5.2

**Search Terms:extended function type restriction

Code

class C
{
    name: string;
    constructor(private handler: F)
    {
        this.name = handler.name;
    }
}

type F = (...args: any[]) => any;

type F2 = F & { inject?: string[] }

function f(x: C | F2):C
{
    if (typeof x == 'function')
        x = new C(f);

    return x;
}

Expected behavior: x in function f should be of type C at the point of the return statement

Actual behavior: x is considered as C or F2, apparently because of the combined type (it works well if we replace F2 with F)

Playground Link: https://www.typescriptlang.org/play/index.html#code/MYGwhgzhAEDCBQBve1XQHZgLYFMBc0EALgE4CW6A5gNwprAD26xJArsEQyQBQAO5ANzBEc0ABZh0AExA4SBAGIBKOqmRoN0ImLIQAdJlzQAvOMky5B7DloaAvvAfwiAT16iFJ6Nz2+wJSggCSRcAbQBdJRMAPmgQ2mc3DwAmL08AMmhEaAoAKxwOAH4CFgpKCOgnADNWdA4yJmgq7gAPAlhoAB9oBWSlPAR1NDIq71d3BlGWk1MAchq6ogb0WZVNNGnTdBwAdzhuKqUEjRIcIlYSdGgW2jsgA

Related Issues: #27143

[RyanCavanaugh]

@orta looks like we're not recognizing that typeof F2 should be always "function" even though it's in an intersection with something whose typeof result would be "object"

@npenin thanks for the nice repro!

[orta]

Cool makes sense, here's a smaller test case also:

// Callable
type F = (...args: any[]) => any;
// Callable but intersected
type F2 = F & { inject?: string[] }

declare const a: string | F2

if (typeof a == 'function') {
    // only F2
    a
} else {
    // Should be only a string
    a
}