wrong behavior about type parameters of `as` clause in a mapped type

🔎 Search Terms

type parameter as mapped type

🕗 Version & Regression Information

5.7.0-dev.20240821

⏯ Playground Link

https://www.typescriptlang.org/play/?ts=5.7.0-dev.20240821#code/C4TwDgpgBAkgzgZWAJwJYDsDmAeAKgPigF4pcBtAXSggA9gJ0ATOKOFDTSqAfihQFdoALigAzAIYAbOBABQoSFADyAW1TAkaLAGkIIPNToNmUAPYAjAFYQAxsEIkA3rKiuoZbVAxQA1ntOipFDiLPCaHHgeFIS09EwsAtC86BAAbhDIItoUIuTZsgC+srKi-Oh2qKboYnj4ABTAuQCUIo5QjOLA4rlQBVDObnzg0ACCxLCI7Fi1UAD0s+0QohnIEIwubgrQAELjY7HGCciCPHzHwmJSMnMLjEsraxuuW1AAwuNhUziOHV25BWQAOS-cSA6I3RbLZCrdaDVbAfjIaqOPohZRqDRfXT6NognoFQjzKAAAxBxK8LFMGPo6yKQA

💻 Code

type IsString<T> = T[] extends string[] ? true : false
type OmitStringKey<T extends object> = {
    [K in keyof T as IsString<T[K]> extends true ? never: K]: T[K]
}

function f<T>(t: T): { data: T } {
    type A = IsString<T> // deferred
    type B = A extends true ? true : false // deferred
    type C = IsString<{data: T}['data']> // deferred
    return {} as OmitStringKey<{ data: T }> // `data` is omitted
}

🙁 Actual behavior

OmitStringKey<{ data: T }> omit data property. That means IsString<T> extends true in the as clause is true, while T is not decided yet.

🙂 Expected behavior

type evaluation should be deferred

Additional information about the issue

No response

Start by running the linked TypeScript Playground repro and compare the deferred aliases with the mapped type's as clause. Trace the compiler's type-parameter evaluation for OmitStringKey<{ data: T }>; done means the data property is not omitted before T is resolved, with regression coverage added in the relevant TypeScript tests.