Type guard fails for a part of an object

[tomfun]

TypeScript Version: 3.4.5

Search Terms: type guard object destructuring, type guard object key

Code

export type GROUP_TYPE = 'group_1' | 'group_2';

export function isGroup(g: any): g is GROUP_TYPE {
    const allowedGroups: GROUP_TYPE[] = ['group_1', 'group_2'];
    return allowedGroups.indexOf(g) !== -1;
}

function getPermissions(query: { group: GROUP_TYPE }) {/* do something */ }

function goodFunction(query: { group: string }) {
    if (!isGroup(query.group))
        return;

    const { group } = query;

    return getPermissions({ ...query, group });
}

function sameFunction(query: { group: string }) {
    if (!isGroup(query.group))
        return;

    return getPermissions(query); // Error here
    //   _________________^^^^^
/*
Argument of type '{ group: string; }' is not assignable to parameter of type '{ group: GROUP_TYPE; }'.
 ...
*/
}

Expected behavior:
Both functions (goodFunction, sameFunction) should be valid typescript code

Actual behavior:
We see TS error

Argument of type '{ group: string; }' is not assignable to parameter of type '{ group: GROUP_TYPE; }'.
  Types of property 'group' are incompatible.
    Type 'string' is not assignable to type 'GROUP_TYPE'.

it thinks that query.group is steel a string when passing as arguments, but knows it is a GROUP_TYPE when gets it from object.

Playground Link:
link to playground

[jack-williams]

Type guards do not propagate type narrowings to parent objects. The narrowing is only applied upon access of the narrowed property which is why the destructing function works, but the reference function does not. Narrowing the parent would involve synthesizing new types which would be expensive. More detail in this comment.

[typescript-bot]

This issue has been marked as a 'Duplicate' and has seen no recent activity. It has been automatically closed for house-keeping purposes.