[BUG][typescript-angular] `anyOf` generates empty `interface` model

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• Have you tested with the latest master to confirm the issue still exists?
• Have you searched for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Sponsorship to speed up the bug fix or feature request (example)

Description

anyOf schema definitions generates an empty interface model.

{
  "components": {
    "schemas": {
      "foo": {
        "type": "object",
        "properties": {
          "bar": {
            "anyOf": [{ "type": "number" }, { "type": "string" }] // <------- this
          }
        }
      }
    }
  }
}

Generates this model

export interface FooBar {  // <--- this doesn't represent type `number | string`
}

import { FooBar } from './fooBar';

export interface Foo { 
    bar?: FooBar;
}

openapi-generator version

This is a regression starting from v6.0.0, it worked in v5.4.0

Output v6.1.0
Output v5.4.0

OpenAPI declaration file content or url

https://github.com/snebjorn/openapi-gen-bug/blob/master/anyof-bug.json

{
  "openapi": "3.0.0",
  "info": {
    "title": "",
    "version": ""
  },
  "paths": {
    "/Foos": {
      "get": {
        "responses": {
          "200": {
            "description": "",
            "content": {
              "application/json": {
                "schema": {
                  "$ref": "#/components/schemas/foo"
                }
              }
            }
          }
        }
      }
    }
  },
  "components": {
    "schemas": {
      "foo": {
        "type": "object",
        "properties": {
          "bar": {
            "anyOf": [{ "type": "number" }, { "type": "string" }],
            "description": "This is the description for bar"
          }
        }
      }
    }
  }
}

Generation Details

Generated using only default values, see https://github.com/snebjorn/openapi-gen-bug/blob/59eb4ba55e91f8444add450016fbd5666c04e2ee/openapitools.json#L25-L29

Steps to reproduce

1. Clone https://github.com/snebjorn/openapi-gen-bug
2. Run yarn install
3. Run yarn gen
4. Observe empty interface in https://github.com/snebjorn/openapi-gen-bug/blob/master/output-6.1.0/anyof-bug/model/fooBar.ts

Related issues/PRs

Couldn't find any related to empty interfaces but a lot of issues popped up regarding anyOf, allOf and oneOf

---- UPDATED ----
This issue was just reported:

• #13538

Suggest a fix

Not exactly sure why bar?: number | string | null; was changed to bar?: FooBar;. Interfaces can't extend primitive types.

// An interface cannot extend a primitive type like 'string'; an interface can only extend named types and classes ts(2840)
interface FooBar extends string, number {}

It could be turned into a type

export type FooBar = string | number;

However there is also the issue of the description. The description is moved to the FooBar interface/type, but the description is describing the property bar which now looks like this

export interface Foo { 
    bar?: FooBar; // <--- note the missing description. The description is on `FooBar` which I believe is incorrect.
}

It should look like this

export interface Foo { 
    /**
     * This is the description for bar
     */
    bar?: FooBar;
}

Start with anyof-bug.json and reproduce the generation using yarn install followed by yarn gen, then inspect output-6.1.0/anyof-bug/model/fooBar.ts and the generated Foo model. Compare the result with output-5.4.0 and confirm that anyOf produces a number|string type while retaining the bar property description on Foo.