No type cast of mapped interface arguments

[johnthecat]

My current task is writing typings for library, that makes all interface sync methods as async. I found how to cast method result, but I didn't found way to cast argument types.

TypeScript Version: 2.5.3
Code

interface Test {
    method(arg: number /* always 1 argument */): string;
}

type Promisify<T> = {
    [P in keyof T]: (/* cast arguments from T[P] somehow */) => Promise<P[T] /* don't know how it works, maybe it's a bug */>;
}

const test: Promisify<Test>;

test.method(256 /* no autocomplete */).then((data) => {
    console.log(
        data.charCodeAt(0); // works right
    );
});

Expected behavior:
User can get method argument types.

Actual behavior:
Typescript should have some special key for getting method arguments,
something like (args: argumentsof T[P]).

[aluanhaddad]

This is not currently supported. Obviously it's very useful behavior and #6606 tracks adding it to the language. I'm not sure why it currently works for return types as the syntax doesn't make any sense. It may be a bug as you say.

[johnthecat]

@aluanhaddad Linked proposal looks good, but it’s exactly what I want.
In my example with this proposal it should looks like
’’’ts
type Promisify = {
[K in keyof T]: (a: typeof P[T][“arguments”][0] /* wtf? */) => Promise<P[T]>
}
’’’

[mhegazy]

#6606 tracks getting the return type, i.e. typeof P[T]()
#12265 tracks the arguments part.

[mhegazy]

Automatically closing this issue for housekeeping purposes. The issue labels indicate that it is unactionable at the moment or has already been addressed.