[BUG][typescript-fetch] When using a openapi.json with AnyOf different date formats, invalid TS is generated

Created by: JakeSummers

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• What's the version of OpenAPI Generator used?
• Have you search for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Bounty to sponsor the fix (example)

Description

Using this JSON:

{
    "openapi": "3.0.2",
    "info": {
        "title": "My Application",
        "version": "0.1.0"
    },
    "paths": {
        "/a/": {
            "get": {
                "responses": {
                    "200": {
                        "description": "Successful Response",
                        "content": {
                            "application/json": {
                                "schema": {
                                    "$ref": "#/components/schemas/Point"
                                }
                            }
                        }
                    }
                }
            }
        }
    },
    "components": {
        "schemas": {
            "Point": {
                "title": "Point",
                "required": [
                    "datetime",
                    "value"
                ],
                "type": "object",
                "properties": {
                    "datetime": {
                        "title": "Datetime",
                        "anyOf": [
                            {
                                "type": "string",
                                "format": "date"
                            },
                            {
                                "type": "string",
                                "format": "date-time"
                            },
                            {
                                "type": "string",
                                "format": "time"
                            }
                        ]
                    },
                    "value": {
                        "title": "Value",
                        "type": "number"
                    }
                }
            }
        }
    }
}

When I run generate --input-spec ./src/generatedApis/openapi.json --generator-name typescript-fetch --output src/generatedApis/typescript-fetch, it will generate invalid Typescript. This is the TS generated:

// tslint:disable
// eslint-disable
/**
 * My Application
 * No description provided (generated by Openapi Generator https://github.com/openapitools/openapi-generator)
 *
 * The version of the OpenAPI document: 0.1.0
 * 
 *
 * NOTE: This class is auto generated by OpenAPI Generator (https://openapi-generator.tech).
 * https://openapi-generator.tech
 * Do not edit the class manually.
 */

import { exists, mapValues } from '../runtime';
/**
 * 
 * @export
 * @interface Point
 */
export interface Point {
    /**
     * 
     * @type {string | Date}
     * @memberof Point
     */
    datetime: string | Date;
    /**
     * 
     * @type {number}
     * @memberof Point
     */
    value: number;
}

export function PointFromJSON(json: any): Point {
    return PointFromJSONTyped(json, false);
}

export function PointFromJSONTyped(json: any, ignoreDiscriminator: boolean): Point {
    if ((json === undefined) || (json === null)) {
        return json;
    }
    return {
        
        'datetime': string | DateFromJSON(json['datetime']),
        'value': json['value'],
    };
}

export function PointToJSON(value?: Point | null): any {
    if (value === undefined) {
        return undefined;
    }
    if (value === null) {
        return null;
    }
    return {
        
        'datetime': string | DateToJSON(value.datetime),
        'value': value.value,
    };
}

This has two main problems:

1. This line: 'datetime': string | DateFromJSON(json['datetime']), is invalid. string is a type not a value.
2. DateFromJSON and DateToJSON are never imported

Full errors from Typescript Compiler

/../Point.ts
Error:(46, 9) TS2322: Type 'number' is not assignable to type 'string | Date'.
Error:(46, 21) TS2693: 'string' only refers to a type, but is being used as a value here.
Error:(46, 30) TS2304: Cannot find name 'DateFromJSON'.
Error:(60, 21) TS2693: 'string' only refers to a type, but is being used as a value here.
Error:(60, 30) TS2304: Cannot find name 'DateToJSON'.

openapi-generator version

The version of my Jar is: 4.2.1

Command line used for generation

Generated using:

`npm bin`/openapi-generator generate --input-spec ./src/generatedApis/openapi.json --generator-name typescript-fetch --output src/generatedApis/typescript-fetch

Using: https://www.npmjs.com/package/@openapitools/openapi-generator-cli

Related issues/PRs

Maybe related: #4340

This issue changed how anyOf was handled: #4387