Union type does not match conditional type

[kevinbeal]

TypeScript Version: 2.9.0-dev.20180501

Search Terms:
union type does not match conditional type

Code

function test<T>(arg: T): T extends any[] ? any[] : number {
    return Array.isArray(arg) ? [] : 1;
}

Expected behavior:
No errors and for the result of test() to be known as array or number.

It's possible there is a good reason for this behavior, but it certainly seems like it should work, to me (with Typescript as my first type annotated language).

Actual behavior:
Error:
Type 'undefined[] | 1' is not assignable to type 'T extends any[] ? any[] : number'.
Type 'undefined[]' is not assignable to type 'T extends any[] ? any[] : number'.

Playground Link: https://www.typescriptlang.org/play/#src=function%20test%3CT%3E(arg%3A%20T)%3A%20T%20extends%20any%5B%5D%20%3F%20any%5B%5D%20%3A%20number%20%7B%0D%0A%20%20%20%20return%20Array.isArray(arg)%20%3F%20%5B%5D%20%3A%201%3B%0D%0A%7D

Related Issues:
Unknown?

[marcellerusu]

Afaik, ternary operator doesn't work on type level. It also doesn't make much sense what it'd actually do here imo.

What's wrong with doing it like

function test(arg: any[] | number): any[] | number {
    return Array.isArray(arg) ? [] : 1;
}

--
EDIT: ignore me, didn't realize type level ternary exists

[mhegazy]

Similar to #22984. This is a design limitation of the current implementation, the relationship between the resulting types and the conditional type is lost through the ternary operation. the compiler can not verify the relationship at the moment.

The alternative is to use overloads to model your example.

function test(arg: any[]): any[];
function test(arg: number): number;
function test(arg: any[] | number): any[] | number {
    return Array.isArray(arg) ? [] : 1;
}

[kevinbeal]

@MarcelRusu

Conditional types (with ternary expressions) were added in 2.8

I have a situation where I have this interface:

interface ControlResult<T> {
	fields: ConvertPrimitiveTo<T, FormControl>;
	controls: T extends any[] ? FormArray : FormGroup;
}

I want variables implementing this interface to be aware of the controls field is specifically being FormGroup or FormArray.

[kevinbeal]

@mhegazy Okay, you can close this then, but it doesn't work for my actual use case.

I just offered a simplified example.

Is another implementation planned?

[mhegazy]

This example might be related to #23132.

[mhegazy]

Is another implementation planned?

we continue to add new inferences, so it is possible.

[mhegazy]

Depending on how you are using ControlResult, an alternative in your other example is to lift the union type over your whole type.

[typescript-bot]

Automatically closing this issue for housekeeping purposes. The issue labels indicate that it is unactionable at the moment or has already been addressed.