Mapped type reported as incompatible when passed through another mapped type (5.4 regression)

[dragomirtitian]

🔎 Search Terms

mapped type assignability regression

🕗 Version & Regression Information

• This changed between versions 5.3 and 5.4

⏯ Playground Link

Playground Link

💻 Code

type ValueType = string;

type Box<T extends ValueType> = { v: T };

type Test<T extends ValueType[]> = T

type UnboxArray<T> = {
 [K in keyof T]: T[K] extends Box<infer R> ? R : never;
};

type Identity<T> = { [K in keyof T]: T[K] };


declare function fnBad<T extends Array<Box<ValueType>>>(...args: T): Test<Identity<UnboxArray<T>>>;

declare function fnOk<T extends Array<Box<ValueType>>>(...args: T): Test<UnboxArray<T>>;


type Unbox<T> = T extends Array<Box<infer R>> ? Array<R> : never;
declare function fnSimilar<T extends Array<Box<ValueType>>>(...args: T): Test<Identity<Unbox<T>>>;

🙁 Actual behavior

Test<Identity<UnboxArray<T>>> is being reported as an error because Identity<UnboxArray<T>> is not assignable to the constraint of Test. Even more strange the error elaboration says Type 'Box<string>' is not assignable to type 'string' which is strange since the type that comes out of UnboxArray<T> can't have Box<string> as an item type. If we don't pass the type though the Identity mapped type, then UnboxArray<T> is assignable to ValueType[].

🙂 Expected behavior

Test<Identity<UnboxArray<T>>> should not be an error as it is in 5.3

Additional information about the issue

No response

[jakebailey]

Bisects to #57122. Anders Hejlsberg (@ahejlsberg)