Function cannot be contextually typed when its rest parameter type is tuple type with leading rest elements

[uhyo]

Bug Report

🔎 Search Terms

function rest arguments parameter tuple type leading middle rest elements

🕗 Version & Regression Information

4.2.3, 4.3.5, 4.4.2

• I was unable to test this on prior versions because leading rest elements in tuple types were introduced in TS 4.2

⏯ Playground Link

Playground link with relevant code

💻 Code

type Func = (...args: [...strs: string[], num1: number, num2: number]) => void

// OK
const func: Func = () => {}
// Error:
// Type '(arg1: string | number) => void' is not assignable to type 'Func'.
//  Types of parameters 'arg1' and 'strs' are incompatible.
//    Type '[...strs: string[], num1: number, num2: number]' is not assignable to type '[arg1: string | number]'.
//      Source has 2 element(s) but target allows only 1.
const func1: Func = (arg1) => {}

// Error:
// Type '(arg2: string | number, arg3: string | number) => void' is not assignable to type 'Func'.
//   Types of parameters 'arg2' and 'strs' are incompatible.
//     Type '[...strs: string[], num1: number, num2: number]' is not assignable to type '[arg2: string | number, arg3: string | number]'.
//       Target allows only 2 element(s) but source may have more.
const func2: Func = (arg2, arg3) => {};

// Error:
// Type '(arg4: string | number, arg5: string | number, arg6: string | number) => void' is not assignable to type 'Func'.
//   Types of parameters 'arg4' and 'strs' are incompatible.
//     Type '[...strs: string[], num1: number, num2: number]' is not assignable to type '[arg4: string | number, arg5: string | number, arg6: string | number]'.
//       Target requires 3 element(s) but source may have fewer.
const func3: Func = (arg4, arg5, arg6) => {};

🙁 Actual behavior

Functions that have one or more arguments (func1, func2, func3) cannot be assigned to variables of type Func.

🙂 Expected behavior

No compiler errors and the types of arguments are inferred as:

• arg1, arg2, arg3, arg4, arg5: string | number
• arg6: string | number | undefined

[jcalz]

Assuming we want to support such assignments, I think you don't want

• arg6: string | number
• arg4: string

If func3 is a Func then you should be able to call func3(1, 2), so arg4 should be string | number while arg6 should be string | number | undefined.

[uhyo]

Thank you for pointing out 😃 you're right, I updated my issue comment.

[graphemecluster]

Although we can wrap all the arguments inside a tuple with rest syntax as a workaround, the type of arg6 is inferred incorrectly to string | number, which lacks undefined:

type Func = (...args: [...strs: string[], num1: number, num2: number]) => void

const func: Func = () => {}

const func1: Func = (...[arg1]) => {}

const func2: Func = (...[arg2, arg3]) => {};

const func3: Func = (...[arg4, arg5, arg6]) => {};

Playground link with the above code

---

Edit: The above code works because the arguments are being inferred as a whole. Adding types makes it break:

type Func = (...args: [...strs: string[], num1: number, num2: number]) => void

// OK
const func: Func = () => {}
// Error:
// Type '(__0_0: string | number) => void' is not assignable to type 'Func'.
//   Types of parameters '__0_0' and 'strs' are incompatible.
//     Type '[...strs: string[], num1: number, num2: number]' is not assignable to type '[string | number]'.
//       Source has 2 element(s) but target allows only 1.
const func1: Func = (...[arg1]: [string | number]) => {}
// Error:
// Type '(__0_0: string | number, __0_1: string | number) => void' is not assignable to type 'Func'.
//   Types of parameters '__0_0' and 'strs' are incompatible.
//     Type '[...strs: string[], num1: number, num2: number]' is not assignable to type '[string | number, string | number]'.
//       Target allows only 2 element(s) but source may have more.
const func2: Func = (...[arg2, arg3]: [string | number, string | number]) => {};
// Error:
// Type '(__0_0: string | number, __0_1: string | number, __0_2: string | number | undefined) => void' is not assignable to type 'Func'.
//   Types of parameters '__0_0' and 'strs' are incompatible.
//     Type '[...strs: string[], num1: number, num2: number]' is not assignable to type '[string | number, string | number, string | number | undefined]'.
//       Target requires 3 element(s) but source may have fewer.
const func3: Func = (...[arg4, arg5, arg6]: [string | number, string | number, string | number | undefined]) => {};

Playground link with the above code

In any case, this issue should be a bug, not a suggestion.

[Andarist]

I'm fixing the primary problem reported in this issue here: #49218 .

That PR doesn't solve the missing | undefined problem though. I think this is a separate discussion and should be handled in a separate PR. In fact, this problem has its own dedicated issue: #46063 (also reported by @uhyo )