spread error when using tuples within overload signatures

[Enteleform]

Bug Report

🔎 Search Terms

• overload
• spread

⏯ Playground Link

Playground link with relevant code

💻 Code

type A = [number, number]
type B = [string, string]
type AB = A | B

function foo(...args:A): void /* works when removed */
function foo(...args:B): void /* works when removed */
function foo(...args:AB){}

const args = [1, 2] as AB

foo(...args)
// A spread argument must either have a tuple type or be passed to a rest parameter.(2556)

🙁 Actual behavior

Type Error:

A spread argument must either have a tuple type or be passed to a rest parameter.(2556)

🙂 Expected behavior

No type errors. All resolutions of the AB union are valid tuples and should be spreadable.

[jcalz]

Essentially a duplicate of #14107

[mkantor]

Even if this continues to be an error, the message is pretty confusing. "A spread argument must either have a tuple type or be passed to a rest parameter"… but args is a union of tuple types and it is being passed to a rest parameter.

(I guess pedantically-speaking a union of tuple types is not "a tuple type", and arguably a rest parameter with a fixed length isn't a "real" rest parameter. But removing the overload signatures makes this error go away, so neither of those things are really the problem here.)

I think ideally the error message would say something about ...args not satisfying any individual overload signature.

[fatcerberus]

arguably a rest parameter with a fixed length isn't a "real" rest parameter

Fun fact, this isn’t just a “no true Scotsman” quibble - IIRC TS sees no difference between (...args: [ number, number ]) and (arg1: number, arg2: number) at the type level. As far as the compiler is concerned there is no rest parameter, “real” or otherwise. This lets you use tuple types to express parameter lists generically.

That said, I’ve agreed for a while that the wording of this error message is confusing in many of the cases where it shows up.

[RyanCavanaugh]

Agreed; if someone wants to update the error message that'd be fine

[typescript-bot]

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