Inserting intersections with 'Function' while narrowing breaks code

[DanielRosenwasser]

@ahejlsberg and I ran into something like this in the RWC. This currently repros on the nightlies.

function foo(x: Object | (() => string)) {
    if (typeof x === "function") {
        // Previously had no errors
        x();
    }
}

Expected: No error

Actual:

[ts] Cannot invoke an expression whose type lacks a call signature. Type '(() => string) | (Object & Function)' has no compatible call signatures.

Potentially related to #25243.

[weswigham]

Mmmm kinda? Previously we only selected applicable union members, while now we also narrow the not-immediately-applicable ones like we do in the non-union case. In the case of primitives and literals, they evaporate into nevers and narrow away, while Object narrows to Object & Function (which would reduce to just Function if we simplified non-primitives) which, given our inability to consider function-ish things as actually potentially callable, causes the issue. So it's an older issue but cropping up in a new place due to the change

[DanielRosenwasser]

What does not immediately applicable mean? Why does this case work?

function foo(x: { blah: number } | (() => string)) {
    if (typeof x === "function") {
        x()
    }
}

[RyanCavanaugh]

Object | (() => string) is subject to subtype reduction so this example is pretty sketchy to begin with.

[weswigham]

So it's probably worth mentioning that Object & Function and () => string on their own are both callable. The first is resolved as an untyped call, while the second is strong. #27422 is related (which is making the error more clear), and so is #7294, #9239, and #10620 (which would involve removing the error).

[rafsawicki]

This caught me off guard today, after upgrading to typescript 3.1. I think it's not uncommon to have a parameter which is either a value or a function returning it - in our company we use it often for helper functions in tests. From a logical point of view it's strange that I can't call something I've just checked to be a function, especially since as @DanielRosenwasser showed it still works in some cases. So for me it's definitely a bug and not just something that requires a better error message.