String union types in props inside generic types

TypeScript Version: 3.9.2 & 4.0.0-beta

Code

type A = {
    foo: number
}

type B<T extends string> = {
    [key in T]?: number
}

type C<T extends string> = A & B<T>

const f = <T extends string>(prop: T, value: number): C<T> => {
    const c: C<T> = {
        foo: 6
    }

    c[prop] = value

    return c
}

type Props = 'bar' | 'baz'

const o = f<Props>('bar', 6)

Expected behavior: It should work 😃

Actual behavior: line 16: Type ‘number’ is not assignable to type ‘C<T>[T]’.

Playground Link: click

[fayzzzm]

Could you check this please. playground

[SancheZz]

@fayzzzm It looks like a hardhack