JSONSchemaType<T> fails compilation when T is a generic type

[ob-ivan]

What version of Ajv are you using? Does the issue happen if you use the latest version?

8.12.0

Your typescript code

Playground

import { JSONSchemaType } from 'ajv'

enum Event {
    FOO = 'foo',
    BAR = 'bar',
}

interface Payload {
    [Event.FOO]: {
        alice: string,
    };
    [Event.BAR]: {
        bob: string,
    };
}

type PayloadSchema = {
    [event in Event]: JSONSchemaType<Payload[event]>;
};

const payloadSchema: PayloadSchema = {
    [Event.FOO]: {
        type: 'object',
        properties: {
            alice: { type: 'string' },
        },
        required: ['alice'],
    },
    [Event.BAR]: {
        type: 'object',
        properties: {
            bob: { type: 'string' },
        },
        required: ['bob'],
    },
}

interface Message<E extends Event> {
    event: E;
    payload: PayloadSchema[E];
}

type MessageSchema<E extends Event> = JSONSchemaType<Message<E>>;

export const getMessageSchema = <E extends Event>(
    eventType: E
): MessageSchema<E> => {
    return {
        type: 'object',
        properties: {
            event: { type: 'string', enum: [eventType] },
            payload: payloadSchema[eventType],
        },
        required: ['event', 'payload'],
    };
};

Typescript compiler error messages

Type '{ type: "object"; properties: { payload: PayloadSchema[E]; }; required: "payload"[]; }' is not assignable to type 'MessageSchema<E>'.
  Object literal may only specify known properties, and 'type' does not exist in type 'never'.

Describe the change that should be made to address the issue?

JSONSchemaType should recognize that the type Message<E> is an object type.

Adding the following test:

type Test<E extends Event> = MessageSchema<E> extends Record<string, any> ? true : false;

export const getMessageSchema = <E extends Event>(
    eventType: E
): MessageSchema<E> => {
    const test: Test<E> = true;
    ...
}

reveals that the constraint MessageSchema<E> extends Record<string, any> is satisfied, and I expect the branch of UncheckedJSONSchemaType that checks T extends Record<string, any> to match and result in the schema type for records and dictionaries.

Are you going to resolve the issue?

Not yet.

[epoberezkin]

I was never supported.

[epoberezkin]

closed accidentally. don't think it's possible to support though.