Type "is assignable to the constraint of type 'T', but 'T' could be instant-CodePudding

I'm struggling with this error message in TypeScript. I've looked at the following SO answers and I simply don't understand the solutions.

Here is a stripped down version of my code with the error:

interface R {
    field: string;
    operator: string;
    value: any;
}
interface RG {
    combinator: 'and'|'or';
    rules: (R|RG)[];
}
interface RGIC {
    rules: (RGIC|R|string)[];
}
type RGT = RG | RGIC;

const f = <T extends RGT>(r: T): T => {
    if ('combinator' in r) {
        return { combinator: 'and', rules: [] }; // <-- TS error here
    }
    return { rules: [] }; // <-- TS error here
}

What I want to express is that if r is of type RG, then function f will return an object that is also type RG. If r is of type RGIC, then function f will return an object that is also type RGIC. But this error is confounding me.

TS Playground link.

CodePudding user response：

The problem you're running into is that generic constraints of the form T extends RGT does not mean "T must be exactly either RG or RGIC". All it implies is that T is compatible with RGT, meaning that it is a subtype of or assignable to RGT. There are subtypes of RGT which are more specific than either RG or RGIC, such as:

interface Whaa {
    combinator: 'or',
    rules: R[],
    brainCellCount: number;
}

const whaa: Whaa = {
    combinator: 'or',
    rules: [],
    brainCellCount: 86e9
}

The Whaa interface is assignable to RG; every Whaa is a valid RG. But a Whaa has a combinator which must be "or", and it also has an additional brainCellCount property of type number.

If your function f has a call signature like this:

declare const f: <T extends RGT>(r: T) => T;

you're saying that the return type of f will be exactly the same as the type of the passed-in r. And therefore the following call will produce a result of type Whaa also:

const hmm = f(whaa);
// const hmm: Whaa

And if so, then this will be fine:

hmm.brainCellCount.toFixed(2); // no compiler error
// BUT AT RUNTIME: