Implicitly type an action payload by an action type

ImplicityTypedActionPayloadByActionType.ts

type ActionTypes = 'actionA' | 'actionB' | 'actionC' | 'actionNoPayload';

type ActionPayloads = {
  actionA: { payload: number };
  actionB: { payload: string };
  actionC: { foo: boolean; bar: boolean };
};

type Actions = {
  [key in ActionTypes]: {
    type: key;
  } & (key extends keyof ActionPayloads ? ActionPayloads[key] : {})
};

type Action = Actions[keyof Actions];

// Pass
const testA: Action = { type: 'actionA', payload: 1 };
const testB: Action = { type: 'actionB', payload: 'string' };
const testC: Action = { type: 'actionC', foo: true, bar: false };
const testD: Action = { type: 'actionNoPayload' };

// Fail
const testUnexpectedActionTypeError: Action = { type: 'not typed' };
const testUnexpectedPayloadError: Action = { type: 'actionNoPayload', payload: 'error' };
const testExpectedPayloadError: Action = { type: 'actionA' };
const testUnexpectedPayloadValueError: Action = { type: 'actionA', payload: 'error' };