Cant infer type of parent based on member of child

[bennetthardwick]

TypeScript Version: 3.4.5

Search Terms:

• infer parent based on child
• type inference in if statement
• conditional generic type infer

Code

const enum ChildType {
    A,
    B,
    C
}

interface ChildrenMap {
    [ChildType.A]: { value: 10 },
    [ChildType.B]: { value: string },
    [ChildType.C]: { value: boolean, test?: boolean }
};

// Create a "Child", that has a type and
// other members based on what type it is
type Child<T extends ChildType> = { type: T } & ChildrenMap[T];

interface ParentMap {
    [ChildType.A]: { color: 'red' },
    [ChildType.B]: { color: 'blue' },
    [ChildType.C]: { color: 'green' }
}

// Create a "Parent", that has a "Child", which
// has a type, which decides what members the 
// parent should have
type Parent<T extends Child<any>> =
    T extends Child<infer R>
    ? R extends ChildType ? ParentMap[R] & { child: Child<R> }
    : never 
    : never;

// More of the same stuff!
interface GrandParent<T extends Parent<Child<any>>> {
    parent: T extends Parent<Child<infer R>>
    ? R extends ChildType ? Parent<Child<R>>
    : never
    : never; 
}

// It all works as expected at this point
let A: Parent<Child<ChildType.A>>;
let red: typeof A['color'] = 'red';
let justRed: typeof A['color'] = 'blue'; // error!

// Same sorta thing as ^
let B: Parent<Child<ChildType.B>>;
let C: Parent<Child<ChildType.C>>;

function testParent<T extends Parent<any>>(parent: T) {
    if (parent.child.type === ChildType.A) {
        parent.child.value; // equals 10!
        parent.color; // red | green | blue, but should only be red
    }
}

function testGrandParent<T extends GrandParent<any>>(grandParent: T) {
    if (grandParent.parent.child.type === ChildType.B) {
        grandParent.parent.child.value; // string, yay!
        grandParent.parent.color; // red | green | blue, but should only be blue
    }
}

Expected behavior: As the type of "Child" is being set using the same conditional type as the parent, the parent type should also get the appropriate type for "color". In the case of testParent, the statement parent.type === ChildType.A correctly infers the value of value (in the case of that example 10). This should also happen for the top level member "color" as well (in the case of that example, color should be set to "red").

Actual behavior: parent.child.type and parent.child.value, correctly coerce the appropriate type. However color is incorrect "red" | "blue" | "green" instead of one of the three.

Playground Link:

link to the playground

Related Issues:

[RyanCavanaugh]

Kind of similar to #31613