Variables (extracted from a discriminated union object) lose narrowed types after being exported

[laam-egg]

🔎 Search Terms

discriminate union export destructure destructuring

🕗 Version & Regression Information

• This is the behavior in every version I tried (including the nightly one), and I reviewed the FAQ for entries about (export, destructure, discriminate, union).

⏯ Playground Link

https://www.typescriptlang.org/play/?#code/GYVwdgxgLglg9mABAQwhApgBygZSgJxjAHMB5MAGwE8AKAfQA8AuRAZwKOIEpEBvRAL4AoUJFgIUaLFAByIChQCCYACYBVVemBF0K8tXrNEYeRUQAfROBVadKnv2Ejw0eEgC2IKCGQKqAUTBkACMKXQAFZBh8AB4AFQA+GgA3XxB0FjiuFl4hRER0INDdFgJ0gBo8xFSKdJYZBDkFELD4hMqBCz4qwpaSxGBfVnRK-Jq641Mu61swXQ7u-JhgRBoTBUQAXm3qtPQLSxntOZUtnfH0Byr8-HRvfCR+XuKVFkGKYfLd2ozJjYEANxVTroD77XL5G53EAPPgFIphV6IMojb7pQRA-LCJwQBDsOHPREsfxfC4sABqgi2iE83l81ECfRUkWiNAARKw4O59uxCCQ0eg2VwgUJlqt-Fd8uSAYgAPSyxAAC3Qt0QMCgatYbA4JC+cAA1lVUBhsHg+WRKLRycKhCCwYtENK5Qrlar1Zq-mZDppjvNEAajVJsE0lKoNDZfXpLTRrUCnOgGJg4PgNbiwPingi-RcqZsaV4fH5GS8Wfh2ZzudrzQKhUJRSsaITdJKBTL5UqVft3TAtbzOF11l6rD67F8AO7oaIqQMm3A6i0GC7C52IfwAJTXpDXtoK9ohrZXrq7GqGcA9ff5lkH0xHJxn0hDynUt90+loS7bCvXm+3wiAA

💻 Code

function acceptStringOnly(_x: string) { }
function acceptNullAndUndefinedOnly(_x: null | undefined) { }

function mutuallyEnabledPair<T>(value: T): {
  enabled: true,
  value: NonNullable<T>,
} | {
  enabled: false,
  value: null | undefined,
} {
  if (null === value || undefined === value) {
    return { enabled: false, value: null };
  } else {
    return { enabled: true, value };
  }
}

const { enabled: E, value: V } = mutuallyEnabledPair("some string value");

if (E) {
  V; // here it is string, ok
  acceptStringOnly(V);
} else {
  V; // here it is null | undefined, ok
  acceptNullAndUndefinedOnly(V);
}

export const { enabled, value } = mutuallyEnabledPair("some string value")

if (enabled) {
  value; // here it is string | null | undefined, weird
  acceptStringOnly(value); // ERROR
} else {
  value; // here it also is string | null | undefined
  acceptNullAndUndefinedOnly(value); // ERROR
}

🙁 Actual behavior

When variables are destructured from an object of a discriminated union type and they are exported, those variables lose their narrowed types defined by the discriminated union type. Meanwhile, if the variables are NOT exported, their narrowed types will be preserved, which is the expected behavior, whether the variables are exported or not.

🙂 Expected behavior

Preserve narrowed types for variables that are:

• Destructured from an object of a discriminated union type
• Exported.

Additional information about the issue

I guess this issue may be more or less related to this one.

[MichalMarsalek]

Simplified repro:

declare function mutuallyEnabledPair(): {
  discriminator: true,
  value: string,
} | {
  discriminator: false,
  value: null | undefined,
}

export const { discriminator, value } = mutuallyEnabledPair()

if (discriminator) {
  value;
}

[MichalMarsalek]

In the function getCandidateDiscriminantPropertyAccess, symbol.valueDeclaration is undefined in the export version & isn't undefined without the export. This leads to the narrowing using the discriminant to not work.

https://github.com/microsoft/TypeScript/pull/59673 Seems to fix it, altough I doubt this is a proper solution.