type Dict<T extends string> = { [key in T]: number }
type DictDict<V extends string, T extends string> = { [key in V]: Dict<T> }

function f<V extends string, T extends string>(dd: DictDict<V, T>, k1: V, k2: T): number {
    // Type 'DictDict<V, T>[V][T]' is not assignable to type 'number'.
    return dd[k1][k2]
}

// works
function f1<V extends string, T extends string>(dd: DictDict<V, T>, k1: V, k2: T): number {
    const d: Dict<T> = dd[k1]
    return d[k2]
}