Incorrect inference of mapped substitution type in conditional involving a union of arrays (regression)

🔎 Search Terms

"inference", "conditional type", "substitution type", "mapped type", "type constraint", "array", "union type"

🕗 Version & Regression Information

This changed between versions 5.0.4 and 5.1.6

⏯ Playground Link

https://www.typescriptlang.org/play?ts=5.1.6#code/C4TwDgpgBAShB2BDAthAJlAvFAThRaA9vADYhQAUA3lANoDWAXFAAo6GQ6gDSEIAuswDOwHAEt4AcygBfKAB8oAV3hoIAMwnoAlLX4AofaEhQAYoUIAeACpQIAD2AI0Q3PiKlyFNhwhcQvOSKKmqa8Dp6CrAIKOgAfFj6UMlQtg5Oqq5wSKgYAPxQAOIQwIFClnAAjkpieGg2cQnMVTV1DYbG0MWlfOUwdo7OWTG5Cdg0tNxQElD0fITqsIKz84swk-yyhgD028pCiJIQRuDQAIJYZhaWtADkiLd5-Am7dABEiG8GnVAAQpfmKy0GiIPLMW4AI1usmeUFetA+XyAA

💻 Code

type Renamed = readonly ({ [k: PropertyKey]: string } | undefined)[]

type Foo<T extends readonly (PropertyKey | undefined)[] | Renamed> =
    T extends Renamed ? GetKeys<Required<T>> : Required<T>
//                              ~~~~~~~~~~
// Type 'Required<T>' does not satisfy the constraint 'Renamed'

type GetKeys<R extends Renamed> = { [K in keyof R]: keyof R[K] }

// usage
type A = Foo<['a'?]> // ["a"]
type B = Foo<[{ a?: 'b' }]> // ["a"]

🙁 Actual behavior

TS fails to see that Required<T extends Renamed> extends Renamed.

🙂 Expected behavior

It should compile.

Additional information about the issue

Substitute readonly (PropertyKey | undefined)[] with something that is not an array and the problem disappears.

I get the same error when I do GetKeys<Id<T>> with type Id<T> = { [K in keyof T]: T[K] }.

Extract<Required<T>, Required<Renamed>> silences the error, but Extract<Required<T>, Renamed> does not.

Beginne mit der verknüpften TypeScript Playground-Reproduktion und vergleiche das Verhalten zwischen den Versionen 5.0.4 und 5.1.6. Untersuche die Inferenz bedingter und gemappter Typen für die Vereinigung von Arrays und überprüfe anschließend, dass die bereitgestellten Beispiele Foo, A und B mit den erwarteten Ergebnissen kompiliert werden, ohne Extract zu benötigen.