Some types are incorrectly assignable from a generic distributive conditional type

TypeScript Version: 3.5.1

Search Terms: generic conditional distributive assignable from

Code

type Keys<T extends object> = T extends unknown ? keyof T : never;

function f<T extends object>(keys: keyof T, moreKeys: Keys<T>) {
    keys = moreKeys;
}

// With this instantiation, `keys` is typed 'a',
// and `moreKeys` is typed 'a' | 'b' | 'c'. The latter
// should not be assignable to the former, but it happens
// in the generic context of the function `f`.
f<{ a: any, b: any } | { a: any, c: any }>('a', 'b');

Expected behavior:

Keys<T> should not be assignable to keyof T since Keys<T> is distributive and will produce a different type than keyof T when T is a union type.

Note that the reverse of this assignment, moreKeys = keys, errors for this exact reason; without a concrete T, it’s hard to tell what the relationship should be, so we simply have no relationship in place.

Actual behavior:

The unsafe assignment is allowed.

Playground Link

Comienza reproduciendo el ejemplo genérico de tipo condicional distributivo en el Playground enlazado. Rastrea la ruta de comprobación de tipos para la asignabilidad en el contexto genérico y, después, añade una prueba de regresión que cubra la asignación insegura y el comportamiento de asignación inversa indicado. Se considera terminado cuando la llamada con 'b' se rechaza porque Keys no es asignable a keyof T.