Type inference when using `extend` in a generic is not consistent about typescript HOT 3 CLOSED

I'm confused because you know about const but aren't using it where doing so would give you the result you want.

Inference is presented with a set of candidates plus a constraint and tries to figure out what you wanted. This talk goes into some examples - it doesn't talk about literals but you can see from that talk why inference makes some decisions but not others.

Why adding extends string means that the type is now inferred as const?

Because if you wanted the function to return string when passed "hello", you wouldn't have written the constraint in the first place.

Why using an array changes the behavior back to not using as const?

Because the prior argument no longer applies (e.g. you might have a subtype of array that you're specializing to)

Why booleans gets the const treatment while strings don't?

Booleans are a union but strings aren't

from typescript.

I'm confused because you know about const but aren't using it where doing so would give you the result you want.

I'm not sure how I'm supposed to get the result I want using const, as it seems to be the opposite of what I want.

Imagine this example: function f<T extends number | string>(value: T): T. How do I make it infer f('hello') as string, f(1) as number, and f<'red' | 'blue'>('red') as 'red' | 'blue' ?
In that case, there is a reason to the constraint - I do want to narrow number | string but I want to narrow it to either number or string.

The only solution I was able to figure out is using overloads instead:

function f(value: string): string;
function f(value: number): number;
function f(value: string | number): string | number;

However, this doesn't work for more complicated examples. Consider this:

function f<T extends (object | boolean)[]>(value: T): T;

I would expect f([{name: 'hello', true}]) to be inferred as ({name: 'hello'}|boolean)[] but it is inferred as ({name: 'hello'}|true)[].

Is it possible to type in a way that this would work?

from typescript.

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

from typescript.