“Unused” function in generic interface changes inferred return type of other function

[max-m]

🔎 Search Terms

• extend generic interface inference
• different variations of the issue title

🕗 Version & Regression Information

• This is the behavior in every version I tried (4.0.5 to 5.7.3), and I reviewed the FAQ for entries about type inference on interface Foo<T> { }

⏯ Playground Link

https://www.typescriptlang.org/play/?#code/C4TwDgpgBAshBOBzCAeAKgGigVQHxQF4o0oIAPYCAOwBMBnKAQypAG0BdKAfilYDoBmHJwBcvIdnYBuAFAzyYAPbxgUAJZVK8AGaMAxtACSABUbw6CdPgDeMqFDDxFYMWll2owABbUUATVwAChcoEzMLeH9cAEoxMPNLOCRUIQDcKSgAekyoAGFFAFsC6lVlKHgIAsUAN2hvNQYAGw06xSg9L2ZkTx91Km0ECppPcGhFbSgAAz1FKjpVNHg1RGR4Sbl7ZSjguNMEyIDY0L2I9CgAHygA2QBfOQ0tXQMoE8s0fHJKWgZ40-eoWz2RzOVy3OTZCGQqHQmGwuHwhGIpHI2FyGgQPSNMzQGZzVQVYAAV3gczEVkC1Vc0UI+FekXe7nRmOx7Vm8ygABkINpgHTduFLPMllREBcoFRCY1Guk5Lj2YtlqtCJzubyBfA+MpAgTiXNAhKpdFohlslAAMpeRSS4YAIyMdJQQo0osuBulMiAA

💻 Code

type Merge<T, U> = T extends any[] ? [...T, U] : [T, U];

export interface IParser<T> {
  prop: T;

  then<Y>(p: IParser<Y>): IParser<Merge<T, Y>>; // Comment or remove this line to change the inferred type of `const Trigger`

  or<Y>(p: IParser<Y>): IParser<T | Y>;
}

interface Parser<T> extends IParser<T> {
  prop: T;
}

////////////////////////////////////////////////////////////////////////////////////////////////////////////////

declare const returns: <T>(v: T) => Parser<T>;

declare const LeftParser: IParser<string | null>;

const Trigger = LeftParser.or(returns(null)); // Should be IParser<string | null>

🙁 Actual behavior

The type of const Trigger gets inferred as IParser<string | [null] | null>.

🙂 Expected behavior

The type of const Trigger gets inferred as IParser<string | null> or IParser<unknown>, the [null] should not be part of the type signature.

Additional information about the issue

I’ve been working on custom typings for Masala Parser in version 0.6 and hit this problem.
Changing line 17 to declare const returns: <T>(v: T) => IParser<T>; fixes the problem as well.

The complete code can be found here: https://gist.github.com/max-m/7678f14661bfe3f6028593c96b66c0b5
The official documentation of Masala Parser’s then function can be found here.

[RyanCavanaugh]

This behavior follows from structural inference. To spell it out:

declare const LeftParser: IParser<string | null>;
const p = returns(null);
const Trigger = LeftParser.or(p);

the type of p here is

{
    prop: null;
    then: <Y>(p: IParser<Y>) => IParser<[null, Y]>;
    or: <Y>(p: IParser<Y>) => IParser<Y | null>;
}

So when we're doing structural inference, there's a match on the type of then wherein a valid candidate inference site for T is IParser<Merge<T, Y>> <-> IParser<[null, Y]>, and [null, Y] is inferred into Merge<T, U>, which is a match for extends any[] and it appears that [null] is the thing you can substitute for T to get [null, Y] as an output.

[max-m]

Thank you very much! I had a feeling that it would be something like that, but couldn’t quite put my finger on it.
Changing the Merge type to type Merge<T, U> = T extends [...rest: infer R] ? [...R, U] : [T, U] seems to fix the inferred types for all cases.