Compilation error with possibly nullable object

[jurajkocan]

TypeScript Version: 2.3.2

Code

export const Fun1 = (): [string, null] | [null, string] => {
    return [null, 'string'];
};

export const Fun2 = (): string => {
    const [val1, val2] = Fun1();
    if (val1) {
        return val1;
    }
    return val2;
// compiler dosn't know val2 can't be ever null
};

Expected behavior:

No error

Actual behavior:

Type 'string | null' is not assignable to type 'string'.
Type 'null' is not assignable to type 'string'.

Detail

This seems like 10198, i have already new version of typescript but this is still not fixed.

[kitsonk]

It seems like for objects, the compiler properly tracks the types, as this works:

export const Fun3 = (): { val1: string; val2: null } | { val1: null; val2: string } => {
    return { val1: null, val2: 'string' };
};

export const Fun4 = (): string => {
    const { val1, val2 } = Fun3();
    if (val1) {
        return val1;
    }
    return val2;
};

So it seems the compiler doesn't properly track and narrow tuple members in the same way it does object property members.

[mhegazy]

The type of one of the keys is not a discriminant. in both examples, the type of val2 is string|null narrowing val1 does not narrow the type of the return value of func3.

Moreover, it is really hard to for the compiler to build a relationship between two variables in such a destructuring pattern.

see: #14630, #10976, #12263, #10065, and #10846

[mhegazy]

Automatically closing this issue for housekeeping purposes. The issue labels indicate that it is unactionable at the moment or has already been addressed.