Typescript Function Overloading "No overload matches this call"

Question

I actually think this is a bug, but I thought I'd ask. I have a function. If passes a single number, I want to perform an operation that returns two objects. If it receives two numbers, I want to return one object. This looks like this now:

function split (splitAt: number | [number, number]): SomeSplittableObject | [SomeSplittableObject, SomeSplittableObject]

but with overloading I should be able to do this:

function split (splitAt: number): [SomeSplittableObject, SomeSplittableObject]
function split (splitAt: [number, number]): SomeSplittableObject
function split (splitAt: number | [number, number]): SomeSplittableObject | [SomeSplittableObject, SomeSplittableObject] {
    // ... Implementation
}

let splitAt: number | [number, number] //... some variable
split(splitAt)

but I get an in-editor error

No overload matches this call.
  Overload 1 of 2, '(splitAt: [number, number]): SomeSplittableObject', gave the following error.
    Argument of type 'number | [number, number]' is not assignable to parameter of type '[number, number]'.
      Type 'number' is not assignable to type '[number, number]'.
  Overload 2 of 2, '(splitAt: number): [SomeSplittableObject, SomeSplittableObject]', gave the following error.
    Argument of type 'number | [number, number]' is not assignable to parameter of type 'number'.
      Type '[number, number]' is not assignable to type 'number'.

even if I cast splitAt as any in the implementation args I get the error

Why would they be a bug? Your variable is deliberately typed to make overload resolution ambiguous and the implementation signature never counts towards the overload set — Aluan Haddad
– Aluan Haddad, Commented Oct 16, 2019 at 19:01

Sam Herrmann · Accepted Answer · 2019-10-16 20:36:42Z

4

The line that is causing your error is this one:

let splitAt: number | [number, number] //... some variable

With the overloading signature that you have, you are saying that you have two forms of the function split. One that takes a number and the other that takes an array with two numbers. You do not have a signature that takes both types and therefore the compiler is complaining.

split(1) // return type is [SomeSplittableObject, SomeSplittableObject]
split([1,2]) // return type is SomeSplittableObject

let splitAt: number | [number, number] //... some variable
split(splitAt) // return type is ???

Edit

Note that when overloading a function, the implemented function signature is shadowed by the overloads and not visible to the outside. In your case above, you only had 2 options. If you want to add the third option where the input/output is "ambiguous", you need to add the signature as an overload (I used shorter types here for clarity):

function foo(x: number): Foo;
function foo(x: string): Bar;
// Define an overload in which the inputs and outputs can be either type
function foo(x: string | number): Foo | Bar;
// Function signature is shadowed by overloads defined above.
function foo(x: string | number): Foo | Bar { /* ... */ }

edited Oct 16, 2019 at 20:36

answered Oct 16, 2019 at 19:14

Sam Herrmann

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2 Comments

Frank Over a year ago

is function split (splitAt: number | [number, number]): SomeSplittableObject | [SomeSplittableObject, SomeSplittableObject] not a form that takes both?

Sam Herrmann Over a year ago

@FranklinHarvey I added an edit to address your comment.

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