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

Beginnen Sie mit dem verlinkten TypeScript Playground und reduzieren Sie die bereitgestellten generic-union-Beispiele für direct this, aliased this und parameter narrowing. Da im Issue keine Repository-Datei oder kein Test angegeben ist, lokalisieren Sie den Einstiegspunkt für das Narrowing im Compiler und fügen Sie einen Regressionstest hinzu, der die gezeigten Fälle abdeckt. Als erledigt gilt die Aufgabe, wenn das Narrowing von direktem und aliasiertem this konsistente Diagnosen erzeugt.