Combination of generic type and mapped types produce unexpected bahaviour

[liaokaien]

I've tried my best to search for related issues and read the document thoroughly but failed to find any useful information. I'm sorry if this bug turns out to be duplicate or even not a bug at all.

TypeScript Version: 3.5.2

Search Terms: conditional types, mapped types, generic

Code

type Concrete = {
  keyOne: string
  keyTwo: string
}

type ConcreteKey = keyof Concrete

type R<T, N extends keyof T> = {
  [P in keyof T]: P extends N ? true : false
};
// R implement without conditional types
type Q<T, N extends keyof T> = {
  [P in N]: true
} & {
  [P in Exclude<keyof T, N>]: false
};
// Function using a concrete type, works as expected
function fn(keys: ConcreteKey[]) {
  let temp: Partial<R<Concrete, ConcreteKey>> = {}
  keys.forEach(key => temp[key] = true)
}
// Function use generic types, report a weird error:
// Type 'true' is not assignable to type 'K extends K ? true : false'.ts(2322)
function fn2<T, K extends keyof T>(keys: K[]) {
  let temp: Partial<R<T, K>> = {}
  keys.forEach(key => temp[key] = true)
}

Expected behavior:
Any property of type R<T,K> could be assigned to true if it extends type K; Therefore true could be assigned to temp[key];

Actual behavior:
Error in function fn2

Type 'true' is not assignable to type 'K extends K ? true : false'.ts(2322)

I suppose type K extends K ? true : false should be automatically resolved to type true.

Playground Link:
Playground Link

I also test the same function with a concrete type and it works as expected. And even if I implement type R without conditional types (Q in example code), the bug still exists, so I guess there is nothing to do with conditional types, but mapped types and generic type;

[jack-williams]

This is working as intended.

I suppose type K extends K ? true : false should be automatically resolved to type true.

The conditional type is distributive so it’s not possible to correctly predict the behaviour until K is instantiated.

Discussion here #31751.

type R<T, N extends keyof T> = {
  [P in keyof T]: [P] extends [N] ? true : false
};

I think this should work.

[liaokaien]

Thanks for your explanation!

[typescript-bot]

This issue has been marked as 'Question' and has seen no recent activity. It has been automatically closed for house-keeping purposes. If you're still waiting on a response, questions are usually better suited to stackoverflow.

[typescript-bot]

This issue has been marked as 'Question' and has seen no recent activity. It has been automatically closed for house-keeping purposes. If you're still waiting on a response, questions are usually better suited to stackoverflow.

[typescript-bot]

This issue has been marked as 'Question' and has seen no recent activity. It has been automatically closed for house-keeping purposes. If you're still waiting on a response, questions are usually better suited to stackoverflow.