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Compilation error with possibly nullable object #15482
Description
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.
Activity
kitsonk commentedon May 1, 2017
It seems like for objects, the compiler properly tracks the types, as this works:
So it seems the compiler doesn't properly track and narrow tuple members in the same way it does object property members.
mhegazy commentedon May 1, 2017
The type of one of the keys is not a discriminant. in both examples, the type of
val2isstring|nullnarrowingval1does not narrow the type of the return value offunc3.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 commentedon May 19, 2017
Automatically closing this issue for housekeeping purposes. The issue labels indicate that it is unactionable at the moment or has already been addressed.