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Function involving FlatArray is transformed in declaration in an incompatible type. #49572
Description
Bug Report
🔎 Search Terms
FlatArray declaration unknown
🕗 Version & Regression Information
- This changed between versions 4.7.0-dev.20220406 and 4.7.0-dev.20220408 (still present in 4.7.3)
⏯ Playground Link
Playground link with relevant code (target: es2019)
💻 Code
// This function's type is changed on declaration
export const fun = (
subFun: <Collection, Field extends keyof Collection>()
=> FlatArray<Collection[Field], 0>[]) => { };
// This argument should match the type of argument used by fun
const subFunExample = <Collection, Field extends keyof Collection>()
: FlatArray<Collection[Field], 0>[] => [];
// Typescript does not throw any error there
fun(subFunExample);
// This is the type found for fun in the declaration file. It should be equivalent to the type for fun.
const fun_declaration: (subFun: <Collection, Field extends keyof Collection>() => (Collection[Field] extends infer T ? T extends Collection[Field] ? T extends readonly unknown[] ? unknown : T : never : never)[]) => void
= (_) => {};
// Typescript throws an error there.
fun_declaration(subFunExample);🙁 Actual behavior
When the declaration is created, the type of fun is transformed to (subFun: <Collection, Field extends keyof Collection>() => (Collection[Field] extends infer T ? T extends Collection[Field] ? T extends readonly unknown[] ? unknown : T : never : never)[]) => void;.
This declaration type does not recognize arguments that were matched by the original type.
🙂 Expected behavior
No error. Declaration type should be equivalent to the original type: if typescript does not raise an error with the original type, it should raise an error for the declaration type.
I've tried to find a simpler type exhibiting this behavior, but didn't find any.
Activity
weswigham commentedon Jun 16, 2022
Hm, so our old emit was compatible, true, but it didn't execute over unions correctly. The new emit was introduced to distribute over unions correctly, but what I find most concerning is that
unknown[]has appeared out of thin air. It used to beexport declare const fun: (subFun: <Collection, Field extends keyof Collection>() => (Collection[Field] extends readonly (infer InnerArr)[] ? InnerArr : Collection[Field])[]) => voidwhich was correctly generic, if not distributive, however the innerinferhas suddenly evaporated now - that's definitely not right. I'm guessing something is being over-instantiated during construction of the new wrapper in this recursive type alias case.