Overloading function fails with union types

[burtek]

// @strictNullChecks: true

interface Data {
    data: any
}

declare const value: Data | undefined;

function doSthWithValue(v: undefined): undefined;
function doSthWithValue(v: Data): Data;
function doSthWithValue(v?: Data) {
    return v;
}

const a = doSthWithValue(value); // <-- error

EDIT: see comment below, it is not connected to strictNullChecks

Expected behavior:

Should not throw error with strictNullChecks enabled

Actual behavior:

It throws error:

Argument of type 'Data | undefined' is not assignable to parameter of type 'undefined'.
  Type 'Data' is not assignable to type 'undefined'.

Playground Link:

Enable strictNullChecks. The order of overloaded signatures does not change anything

https://www.typescriptlang.org/play/#src=interface%20Data%20%7B%0D%0A%20%20%20%20data%3A%20any%0D%0A%7D%0D%0A%0D%0Adeclare%20const%20value%3A%20Data%20%7C%20undefined%3B%0D%0A%0D%0Afunction%20doSthWithValue(v%3A%20undefined)%3A%20undefined%3B%0D%0Afunction%20doSthWithValue(v%3A%20Data)%3A%20Data%3B%0D%0Afunction%20doSthWithValue(v%3F%3A%20Data)%20%7B%0D%0A%20%20%20%20return%20v%3B%0D%0A%7D%0D%0A%0D%0Aconst%20a%20%3D%20doSthWithValue(value)%3B%0D%0A

Related issues:
#14107

[jack-williams]

This isn't actually related to strict null checks, but related to overload resolution. Here is the same problem without optionality and undefined.

declare const value: "L" | "R";

function doSthWithValue(v: "L"): "L";
function doSthWithValue(v: "R"): "R";
function doSthWithValue(v: "L" | "R") {
    return v;
}

// Argument of type '"L" | "R"' is not assignable to parameter of type '"R"'.
const a = doSthWithValue(value);

I think the current situation is that TypeScript looks to prune overloads by only looking at the overloads that match the input type. If no overload individually matches then you get a type error. Basically, TypeScript is missing the following distributive rule for intersection types:

(A -> C) & (B -> C) <: (A | B) -> C

[burtek]

For some reason it worked when disabling strictNullChecks, so I thought it might be the issue. But your's example fails also without enabling it. Still a bug, I guess...?

Should I change the name of the issue? To what?

[jack-williams]

Managed to track down what looks to be the canonical issue for this problem: #14107.

[burtek]

For my case the following is enough:

function doSthWithValue(v: Data): Data;
- function doSthWithValue(v: undefined): undefined;
+ function doSthWithValue(v: Data | undefined): Data | undefined;
function doSthWithValue(v?: Data) {
    return v;
}

Yet the issue is still current. Closing in favour of #14107 with hope for getting it fixed soon