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Conditional types cannot infer args array #23229
Description
TypeScript Version: 2.9.0-dev.20180406
Search Terms:
conditional type infer array parameter
Code
// This works
type PromisifyWithOneArg<T> = {
[P in keyof T]?: T[P] extends (arg: infer R1) => infer R2 ? (args: R1) => Promise<R2> : T[P];
};
// This doesn't
type Promisify<T> = {
[P in keyof T]?: T[P] extends (...args: infer R1) => infer R2 ? (...args: R1) => Promise<R2> : T[P];
};
Expected behavior:
These type definitions work, allowing definition of a generic type that makes all methods in an interface return promises, while preserving the type constraints of the arguments.
Actual behavior:
The ...args version is rejected. I either have to define N versions of the conditional type with different numbers of arguments, or lose type safety for the arguments.
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Related Issues:
Activity
mhegazy commentedon Apr 6, 2018
rest parameters need to be of array type...
general issue of unifying parameters and arrays/tuples is #5453
typescript-bot commentedon Apr 21, 2018
Automatically closing this issue for housekeeping purposes. The issue labels indicate that it is unactionable at the moment or has already been addressed.