Value accessed with generic that extends keyof does not compile when passed to conditional type

[undeadcat]

🔎 Search Terms

keyof, conditional type, generic

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about distributive conditional types (Generic conditional type T extends never ? 'yes' : 'no' resolves to never when T is never. #31751)

⏯ Playground Link

https://www.typescriptlang.org/play/?ts=5.9.3#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

💻 Code

export interface ServiceCredentials {
  ["google"]: {foo:string};
  // more providers here, truncated for brevity
}

export type JSONCompatible<T> = 

  [T] extends [Date]
  //or 
    // T extends Date
  ? never
  // more logic here, truncated for brevity
  : T

export declare function toJson<T>(val: JSONCompatible<T>): void;


function handleSelectedCredential<T extends keyof ServiceCredentials>(credType: T, cred: ServiceCredentials) {
  const thisCredential: ServiceCredentials[T] = cred[credType];

  toJson<ServiceCredentials[T]>(thisCredential);
}

🙁 Actual behavior

Argument of type 'ServiceCredentials[T]' is not assignable to parameter of type 'JSONCompatible<ServiceCredentials[T]>'.
  Type '{ foo: string; }' is not assignable to type 'JSONCompatible<ServiceCredentials[T]>'.(2345)

🙂 Expected behavior

I would expect the file to compile.
This similar code that does not use a generic compiles:

declare const x: keyof ServiceCredentials

function handleSelectedCredential(cred: ServiceCredentials) {
  const thisCredential: ServiceCredentials[typeof x] = cred[x];

  toJson<ServiceCredentials[typeof x]>(thisCredential);
}

Additional information about the issue

Note that I have tried both versions of JSONCompatible, taking into account distributive conditional types:

export type JSONCompatible<T> = [T] extends [Date] ? never : T
export type JSONCompatible<T> = T extends Date ? never : T

The result is the same for me.

The intended behavior of the original version of JSONCompatible is to prohibit values that don't deserialize to the same value (e.g. JSON.parse(JSON.stringify(new Date())), originally it contained more code. This is the minimal example.

[MartinJohns]

This is working as intended / a design limitation.

Resolution of unbound conditional types is deferred. The compiler can't verify if the argument thisCredential is compatible with the invocation of toJson<ServiceCredentials[T]>() because the compiler doesn't know what type T is, so it doesn't know what the type JSONCompatible<ServiceCredentials[T]> is either.

[RyanCavanaugh]

Yeah, there's really not an obvious way to do this that isn't potentially combinatorially explosive.