Narrowing of generic this is inconsistent with variable narrowing

🔎 Search Terms

generic union constraint narrowing this

🕗 Version & Regression Information

• This is the behavior in every version I tried

⏯ Playground Link

https://www.typescriptlang.org/play/?noImplicitReturns=false#code/C4TwDgpgBAKu0F4oEkDMAmKAfKBlYATgNwBQAlgHbAQEBmAhgMbT4FQDeJUUA5gQPYBXMAC4oAIgr8KEcaW4BrSgBMx4gM6E5JAL7kqNBsxQYOXXgOFr1jegBt6BbYpVqyGbXpLKIjBwWhaQQpGYDJpKGB6BQg0dAAKADd7QQgxOIBKMUT+MmVSHz9HQODQ8IooSloaAGkVAB4YAD4klLSOKCUKVVgoHSzYUhIgkLCI6k0YAAsydUaoCAAPam71WHgW4Bn1MRgMs251AHcyYEYpqHit2YA6PiEwfc5ublt1aElpWRFzF6gA4CCAgUeQvN4fGz2RziH5-Q4nM4XK7bG5dZRPX5w8ESTROMQAenxUCkCwIAgIAH5MX9seJ3OgYdS+uY9F4RmVxhBNAAFRz0AC2EGoBHmSxWyjWcEgLX4uwx8NO50u-DulkeBzB9HeEikMkZcP+QqBIMxtMh-n1cOOiqRKrR8oNUDNWgJRJo5IAhEzXlqPvTLX89NxWSRhqUxhUJsAAIJ2Mi+5TTWai5YQVbraXI2ZyjWMaSaKD8KBIa7qUHWxHK1UPB0+7WfPWwuEA42gusQ2wWpt-CtK+J2lS1rG+nEuqCE0me71Okd0jzdl5B5ls8PlSJc4BJ9ToFPiyUbLM7WADOLYKDBHy0SgQZQasi0S6l6vCYsIJAaTvQod5igF5J2VJ0lMEttiIcc3TJfgCExKIYjiQ8MjAid3SgmDtigdR6DCdQry5Ex0CQiDyVNfNgCgRwBCOYtLhPYCmkiUDwMnVC-hbYEGNmQjmOg4NdFDdkI3XHk+UFYUdxgBZU3TKUIBlOUgMwHALwgK8ZFvZ5KgfftnzAV933NL8NVeUioH-QD8Oo-guP4BQYOiWIMH7RCmJszEi0w7DcLWOJrNsmkTIo-gqKQeJaMwBB6Ksly-JeNiKiiidXN4ldRjXKNY3jd5E22cTJL3DNZMPeSLKU7oVOvdTzB-AsixAzjzHvKt7hfN99M-JxvxMsz2lPJAEqJJKXlghyEn4ZzEpi7h3Kw2YvPw3ySN-MjAuCmiFOLSKFtYo12P6wsYpDIA

💻 Code

type Type = I32 | Str;
interface Str {
  group: "none";
  kind: "str";
}
interface I32 {
  group: "scalar";
  kind: "i32";
}

declare function takeI32(value: I32): void;
declare function inferKind<T>(value: { kind: T }): T;

function testThis<T extends Type>(this: T) {
  switch (this.group) {
    case "none":
      return;
    case "scalar":
      switch (this.kind) {
        case "str": // no error?
        case "i32":
      }
  }
}

function testParameter<T extends Type>(o: T) {
  switch (o.group) {
    case "none":
      return;
    case "scalar":
      switch (o.kind) {
        case "str": // error!
        case "i32":
      }
  }
}

function testAliasedThis<T extends Type>(this: T) {
  const o = this;
  switch (o.group) {
    case "none":
      return;
    case "scalar":
      switch (o.kind) {
        case "str": // error!
        case "i32":
      }
  }
}

function testThis2<T extends Type>(this: T): I32 | undefined {
  if (this.group === "scalar") {
    const value: I32 = this; // error
    takeI32(this); // error
    this satisfies I32; // error
    const arrow = (): I32 => this; // error
    return this; // error
  }
}

function testParameter2<T extends Type>(o: T): I32 | undefined {
  if (o.group === "scalar") {
    const value: I32 = o; // ok
    takeI32(o); // ok
    o satisfies I32; // ok
    const arrow = (): I32 => o; // ok
    return o; // ok
  }
}

function testAliasedThis2<T extends Type>(this: T): I32 | undefined {
  const o = this;
  if (o.group === "scalar") {
    const value: I32 = o; // ok
    takeI32(o); // ok
    o satisfies I32; // ok
    const arrow = (): I32 => o; // ok
    return o; // ok
  }
}

🙁 Actual behavior

The errors (or lack of them) are inconsistent between narrowing this directly vs narrowing an "aliased" this (const alias = this;)

🙂 Expected behavior

They should be consistent

Additional information about the issue

No response

Bắt đầu với TypeScript Playground được liên kết và rút gọn các ví dụ generic-union được cung cấp cho direct this, aliased this và parameter narrowing. Issue không xác định file nào trong repository hoặc test nào, vì vậy hãy tìm entry point của compiler narrowing và thêm một regression test bao phủ các trường hợp được trình bày. Hoàn tất khi việc narrowing direct this và aliased this tạo ra các diagnostics nhất quán.