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Unsafe assignment allowed from a function union to a function type with rest parameters #62447
Description
🔎 Search Terms
- function union assign rest parameter
- union of function signatures rest parameter
- function union assignable to rest parameter
- function overload union assign rest
- function union parameter intersection rest assign
- correlated union types
- callable union parameter count
- overload resolution union
🕗 Version & Regression Information
This is the behavior in every version I tried, and I reviewed the FAQ for entries about function union assignability and overload resolution.
⏯ Playground Link
💻 Code
type F = (() => number ) | ((text: string) => number)
const f: F = (text) => text.length
type G = (...args: string[]) => number
const g: G = f
g()🙁 Actual behavior
No type error is reported on the assignment const g: G = f.
This allows a subsequent call g() which is type-safe according to G's signature. However, because the assigned implementation f requires an argument, this valid call leads to a runtime TypeError.
The type system is allowing an assignment that is demonstrably unsafe at runtime.
🙂 Expected behavior
A type error should be reported on the line const g: G = f.
The assignment should be disallowed because not all constituents of the union type F are safely assignable to the type G.
Specifically, the (text: string) => number member of F cannot be safely called with zero arguments, but the target type G explicitly allows being called with zero arguments. Therefore, the assignment creates a situation where a runtime error is inevitable if that code path is taken, which defeats the purpose of static type checking.
Additional information about the issue
I told Gemini to search known issues and found:
Activity
jcalz commentedon Sep 14, 2025
This has little to do with unions; you can expose the same unsoundness with
As such this is a known and intentionally allowed unsoundness, and this issue is a duplicate of, say, #58552