type.ts

type Config<T> =
  T extends { type: 'sftp' } ? { login: string, password: string } :
    T extends { type: 'post' } ? { url: string, header: string } : {};

const union: { type: 'post' | 'sftp' } = { type: 'post' }
const simple: { type: 'post' } = { type: 'post' }
const splitUnion: { type: 'post' } | { type: 'sftp' } = { type: 'post' }
type Instance = Config<typeof union> // ==> Instance is {} and not {url: string, header: string} 
type InstanceOk = Config<typeof simple> // ==> Instance is { url: string, header: string } 
type InstanceOk2 = Config<typeof splitUnion> // ==> Instance is { url: string, header: string }