Cannot type-guard conditional extends WeakRef

[jhunterkohler]

Bug Report

🔎 Search Terms

• WeakRef
• weakreference
• conditional
• conditional type
• conditional not assignable
• return type not assignable
• conditional typeguard
• instanceof
• instanceof typeguard
• instanceof extends

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about:
  • Generics
  • Typeguards

⏯ Playground Link

Playground link with relevant code

💻 Code

/**
 * If `value` is a `WeakRef`, return the dereferenced object, or throw if
 * it has been garbage collected. Else, return `value`. */
function xderef<T>(value: T): T extends WeakRef<infer U> ? U : T {
    if (value instanceof WeakRef) {
        const val = value.deref();

        if (!val) {
            throw new ReferenceError(value);
        }

        return val;
    }

    return value;
}

🙁 Actual behavior

Compiler reports, on the final return statement return value:

Type 'T' is not assignable to type 'T extends WeakRef ? U : T'.

🙂 Expected behavior

Compiler would recognize that T does not extend WeakRef, thus that T must only be compatible with itself.

[jcalz]

duplicate of #33912

[typescript-bot]

This issue has been marked as a 'Duplicate' and has seen no recent activity. It has been automatically closed for house-keeping purposes.