TS2345 error with derivated types from a map of types

[lgraziani2712]

TypeScript Version: 3.7.2

Search Terms: ts2345 derivated type error

I really don't know how to describe this bug in short terms.

Code

enum A {
    a = 'a',
    b = 'b'
}

interface ObjA {
    prop1: string;
}
interface ObjB {
    prop2: number;
}

type PayloadMap = {
    [A.a]: ObjA;
    [A.b]: ObjB;
}

type Content<T extends A> = {
    type: T;
    payload: PayloadMap[T];
}

function processContent<T extends A>(content: Content<T>) {
    parserPayload[content.type](content.payload);
//                              ^=========== here is the error
}

const parserPayload: { [key in A]: (payload: PayloadMap[key]) => any } = {
//                                           ^============ because this is treated
//                                                         as an intersection
    a: payload => payload.prop1,
    b: payload => payload.prop2,
}

Expected behavior: To correctly know that PayloadMap[key] is in fact the same as PayloadMap[T] since key matches T.

Actual behavior:

Argument of type 'PayloadMap[T]' is not assignable to parameter of type 'ObjA & ObjB'.

Playground Link: https://www.typescriptlang.org/play/?ssl=1&ssc=1&pln=30&pc=2#code/KYOwrgtgBAglDeAoKKoEMoF4oHI04BplUAjLXEnRAX0UQEsQAXYAJwDM0BjYKAeRIArOElRQADqwD24gIwAuKAGcmrRgHMA3DQbM2nHvyEAhBMRSSZAJkXgIJNttqImAT3G8ACmlcAbKWgAJgCyaOLkoqgA2jAAdGgAuooCwtpiMbEkSUaCxk50bh5QAMJSeswAPAAqUMAAHiwggUqwAHwR5lCFwIpVaajiPv5Bit5+ASFhUVUJ+YjsYCBcTPRlEtI8Skql5UzVtQ2gzW0AFFxljUyKO5fVrQCUZmKDrEpsY8OBUee7sd0JZwuoCYsUG4yC9zmPxUEjQr3eQwmingUCiAGtgK4oIxYNkTmDPqNEUFQuJ0ZiEo9MO00CAsdQOmI0IoCRMsO1WUFQdI5EQxCQWcTAuzYeDAtzrERqEA

Related Issues:

[fatcerberus]

Indexed access produces intersection type => most likely caused by #30769.

[RyanCavanaugh]

TS doesn't know that this is OK to do. You could have written this, which would be seen equivalently:

function processContent<T extends A>(content1: Content<T>, content2: Content<T>) {
    parserPayload[content1.type](content2.payload);
}
declare const a: Content<A.a>, b: Content<A.b>;
// Causes a runtime error, but is an OK call
processContent(a, b);

[RyanCavanaugh]

Duplicate #31445

[lgraziani2712]

TS doesn't know that this is OK to do. You could have written this, which would be seen equivalently:

BTW, isn't that a kind of bug that TS doesn't check? There are a lot of business logic bugs with correct typings that TS isn't able to catch and that's OK. Isn't the bug you're exemplifying one of that kind?

Thank you for your rapid response!

[RyanCavanaugh]

That's a type error, not a business logic error. You can construct examples that observe a string where a number is expected, for example.