TypeScript narrow non-discriminated union

test.ts

{
    type AncestryType = {
        type: string;
    };

    type AncestryCategory = {
        type: string;
        category: string;
    };

    type AncestrySubcategory = {
        type: string;
        category: string;
        subcategory: string;
    };

    type Ancestry = AncestryType | AncestryCategory | AncestrySubcategory;

    const fn1 = (ancestry: Ancestry) => {
        if (!('category' in ancestry)) {
            ancestry; // ✅ AncestryType
        } else {
            // ✅ AncestryCategory | AncestrySubcategory
            ancestry;
        }
    };

    const checkAncestryIsType = (ancestry: Ancestry): ancestry is AncestryType =>
        !('category' in ancestry);

    const fn2 = (ancestry: Ancestry) => {
        if (checkAncestryIsType(ancestry)) {
            ancestry; // ✅ AncestryType
        } else {
            /*
            ❌
            Actual: never
            Expected: AncestryCategory | AncestrySubcategory
            */
            ancestry;
        }
    };
}

//
// Workaround
// https://github.com/microsoft/TypeScript/issues/32680#issuecomment-517726428
//

{
    type AncestryType = {
        _type: 'type';
        type: string;
    };

    type AncestryCategory = {
        _type: 'category';
        type: string;
        category: string;
    };

    type AncestrySubcategory = {
        _type: 'subcategory';
        type: string;
        category: string;
        subcategory: string;
    };

    type Ancestry = AncestryType | AncestryCategory | AncestrySubcategory;

    const checkAncestryIsType = (ancestry: Ancestry): ancestry is AncestryType =>
        !('category' in ancestry);

    const fn1 = (ancestry: Ancestry) => {
        if (checkAncestryIsType(ancestry)) {
            ancestry; // ✅ AncestryType
        } else {
            ancestry; // ✅ AncestryCategory | AncestrySubcategory
        }
    };
}