TypeScript函数返回类型与泛型不匹配时未抛出预期类型错误

我在TypeScript中实现了registerIpcChannel函数，用于注册带指定处理器的IPC通道，处理器类型需要匹配IpcChannel接口的定义。大部分场景下类型检查正常，但当处理器故意返回undefined或null时，TypeScript未抛出预期的类型错误。

简化代码如下：

/**
 * Interface that describes an IPC channel
 * @template P - Type of the parameters for the handler
 * @template R - Type of the return value for the handler
 */
interface IpcChannel<P extends any[], R> {
  name: string;
}

/**
 * Function that takes in a IpcChannel interface, handler function, registers it
 */
const registerIpcChannel = <P extends any[], R>(
  ipcChannel: IpcChannel<P, R>,
  handler: (...params: P) => R
): void => {
  // Do things...
};

/**
 * Create definition of an IPC channel
 * Accepts a single parameter that is a string, returns a string
 */
const testIpcChannel: IpcChannel<[string], string> = {
  name: '/foo/bar',
};

//
// Testing the register function
//

/**
 * Register the IPC channel
 * Sanity check passes, no type error is thrown.
 */
registerIpcChannel(testIpcChannel, (arg) => {
  return 'This returns a string: ' + arg;
});

/**
 * Test with returning the wrong type
 * Sanity check passes, a type error is thrown because a number is returned.
 */
registerIpcChannel(testIpcChannel, (arg) => {
  return 1;
});

/**
 * Test with returning undefined
 * Sanity check FAILS, this should throw a type error
 */
registerIpcChannel(testIpcChannel, (arg) => {
  return undefined;
});

最后一个返回undefined的registerIpcChannel调用本应触发类型错误，但实际没有。

我尝试将handler的返回类型约束为NonNullable<R>，这样能触发返回undefined/null时的类型错误，但会破坏处理器返回void的场景：

const registerIpcChannel = <P extends any[], R>(
  ipcChannel: IpcChannel<P, R>,
  handler: (...params: P) => NonNullable<R> // + Added `NonNullable`
): void => {
  // Do things...
};

/**
 * Void test
 * Fails, this now throws an error after adding `NonNullable`
 */
const voidIpcChannel: IpcChannel<[string], void> = {
  name: '/foo/bar',
};

registerIpcChannel(voidIpcChannel, (arg) => {
  return;
});

有意思的是，如果把处理器定义移到IpcChannel接口内部，类型检查就能正常工作，但受限于架构，必须让处理器函数与IpcChannel接口分离，所以这种方案不可行：

//
// Handler on the `IpcChannel` interface
//
interface IpcChannel2<P extends any[], R> {
  name: string;
  handler: (...args: P) => R;
}

/**
 * Sanity check passes, a type error is thrown.
 */
const test: IpcChannel2<[string], string> = {
  name: '/foo/bar',
  handler: (arg) => {
    return undefined;
  },
};

内容的提问来源于stack exchange，提问作者upset-toaster