TS3.6/3.7 regression: template arguments used in unions no longer resolve/narrow

[mprobst]

TypeScript Version: 3.7.1-rc

Search Terms: template argument union

Code

declare class Data {
 string<T>(defaultValue?: string|T): string|T;
}

declare const str: string|undefined;
const x = new Data().string(str);

Expected behavior:

I'd argue string, at least that's what the user intended here.

Actual behavior:

In TypeScript version:

• 3.5, typeof x === string
• 3.6, typeof x === unknown
• 3.7, typeof x === string|undefined

Playground Link: n/a

[mprobst]

I presume this is an intentional change (?) but I could find an existing issue.

[AlCalzone]

IMO the inferred type is the only logical one here. You pass a variable of type string|undefined to an argument expecting string|T, so T is inferred as undefined. The method returns string|T, so I expect the return type to be string|undefined.

[mprobst]

AlCalzone note the ? in defaultValue?. The parameter is optional, so its real type is string|T|undefined. We're passing string|undefined into string|undefined|T, so T should/could be never, I guess?

[RyanCavanaugh]

I think this is a bug; undefined should indeed be part of the union for the purposes of inference. So T = never and x should be string. Maybe this is the smallest code snippet that can produce different behavior in 4 consecutive versions of TypeScript 😶

[fatcerberus]

If there's no way to infer T then shouldn't it default to its constraint of unknown (the TS 3.6 behavior), rather than becoming never?

[ahejlsberg]

The core issue here is what, if anything, to infer for T when inferring from a source type X to a target type X | T?

1. Make no inference. Problem here is if we make no other inferences for T, we'll end up with the constraint, which typically is the default unknown. That's a terrible outcome because it is less specific than what we started with.
2. Infer never. This doesn't work if we're inferring for a function that returns just T since it would suddenly imply the function never returns.
3. Infer X. This is pretty much the only meaningful thing to do.

In 3.5 we'd pick option 3 for a non-union type X and option 2 for a union type X. That was obviously inconsistent. In 3.6 we started out with option 1 and then, prompted by issues, switched to option 3 in the 3.6.3 release. Nothing has changed in 3.7 and we're still doing option 3. I continue to think this is the only meaningful option.

[ahejlsberg]

See #32460, #32558, and #33252 for more context.