TypeScript版本： 3.5.3(也尝试过@next)
搜索词： const symbol property "not assignable to type"
代码

/*
* For reasons of how the JSON is converted, I'm using Symbols to hide certain properties.
*/

class Server {
    public auth: string | null = null;
}

/** `tsc` complains about this[AUTH_PROP] even though it must always be a string inside the `if()` */
const AUTH_PROP = Symbol();
class TestSymbol {
  private readonly [AUTH_PROP]: string | null = null;
  public readonly server: Server | null = null;

  public get auth(): string | undefined {
    if (this[AUTH_PROP] !== null) {
      return this[AUTH_PROP];
    }
    if (this.server && this.server.auth) {
      return this.server.auth;
    }
    return undefined;
  }
}

/** same thing with a string literal and it works */
class TestNoSymbol {
    private readonly _auth: string | null = null;
    public readonly server: Server | null = null;

  public get auth(): string | undefined {
    if (this['_auth'] !== null) {
      return this['_auth'];
    }
    if (this.server && this.server.auth) {
      return this.server.auth;
    }
    return undefined;
  }
}

/** same thing with a const string and it fails */
const AUTH_PROP_S = '_auth';
class TestConstString {
    private readonly [AUTH_PROP_S]: string | null = null;
    public readonly server: Server | null = null;

  public get auth(): string | undefined {
    if (this[AUTH_PROP_S] !== null) {
      return this[AUTH_PROP_S];
    }
    if (this.server && this.server.auth) {
      return this.server.auth;
    }
    return undefined;
  }
}

预期行为：

所有这三个版本都应该没问题，不会出现tsc错误。const的值永远不会改变，所以它必须始终是string语句内的if()。

实际行为：

Type 'string | null' is not assignable to type 'string | undefined'.
  Type 'null' is not assignable to type 'string | undefined'.

Playground链接：https://is.gd/V7Jai9
相关问题： 不确定