Typescript: [Bug] [keyof] Unexpected behavior of `keyof` of an interface with optional properties

[franklixuefei]

TypeScript Version: 2.9.1

Search Terms:
keyof,optional
Code

export type Map<T = {}> = {
  readonly [P in Extract<keyof T, string>]: A<P>;
};

export interface A<TNameType extends string = string> {
    test: string;
}

export interface B {
    required: string;
    optional?: string;
}

export const Test: Map<B> = {
    required: 'required',
}

Expected behavior:

No errors should be reported, because an optional property should not be required as a result of keyof.

Actual behavior:

Type '{ required: string; }' is not assignable to type 'Map<B>'.
  Property 'optional' is missing in type '{ required: string; }'.

It was working in 2.8.3, without Extract<>

Playground Link:
Playground Link

Related Issues:

[DanielRosenwasser]

This is pretty unfortunate - our workaround to only include string keys was to use Extract<keyof T, string>, but mapped types that use that trick are no longer considered homomorphic mapped types, which means that we can't preserve modifiers on properties.

[DanielRosenwasser]

Is there a reason you only want to include string keys?

[jcalz]

I think if you use a layer of indirection you can get the desired homomorphicity back (homomorphicness? homomorphologicality?). Specifically, make a new type parameter K extends keyof T, assign Extract<keyof T, string> to it, and then map over that. Like this:

export type Map<T = {}, K extends keyof T = Extract<keyof T, string>> = {
  readonly [P in K]: A<P>;
};

or, equivalently

type OnlyStringKeyProps<T> = Pick<T, Extract<keyof T, string>>; 
export type Map<T = {}> = {
  readonly [P in keyof OnlyStringKeyProps<T>]: A<P>;
};

Then the following should work (note that the type of the required property had to be modified to type {test: string} as defined in A<> ; I assume OP wasn't meaning to make this error):

export const Test: Map<B> = {
  required: { test: 'required' },
};  // no error now

So at least there's a workaround? Not sure if this is the way to do it going forward, or if there are other ideas.

[typescript-bot]

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