[BUG][typescript-fetch] Typescript error in generated code for endpoint returning type boolean

Created by: matthewbpt

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? 4.0.0-beta3 from npm
• Have you search for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Bounty to sponsor the fix (example)

Description

When generating the typescript api client, the generated client has type errors for any methods returning a boolean.

export class UserApi extends runtime.BaseAPI {

    async userEmailExistsRaw(requestParameters: UserEmailExistsRequest): Promise<runtime.ApiResponse<boolean>> {
        if (requestParameters.email === null || requestParameters.email === undefined) {
            throw new runtime.RequiredError('email','Required parameter requestParameters.email was null or undefined when calling userEmailExists.');
        }

        const queryParameters: runtime.HTTPQuery = {};

        const headerParameters: runtime.HTTPHeaders = {};

        const response = await this.request({
            path: `/api/1.0/users/userExists/{email}`.replace(`{${"email"}}`, encodeURIComponent(String(requestParameters.email))),
            method: 'GET',
            headers: headerParameters,
            query: queryParameters,
        });

        return new runtime.TextApiResponse(response);
    }

    async userEmailExists(requestParameters: UserEmailExistsRequest): Promise<boolean> {
        const response = await this.userEmailExistsRaw(requestParameters);
        return await response.value();
    }
}

The return statement from userEmailExistsRaw has error:

Type 'TextApiResponse' is not assignable to type 'ApiResponse<boolean>'.
  Types of property 'value' are incompatible.
    Type '() => Promise<string>' is not assignable to type '() => Promise<boolean>'.
      Type 'Promise<string>' is not assignable to type 'Promise<boolean>'.
        Type 'string' is not assignable to type 'boolean'.ts(2322)

openapi-generator version

4.0.0-beta3 from npm

OpenAPI declaration file content or url

---
swagger: '2.0'
info:
  description: Operations related to the API
  version: '1.0'
  title: API
host: api.example.com
basePath: "/"
tags:
- name: User
  description: All apis relating to Users
schemes:
- http
- https
produces:
- application/xml
- application/json
paths:
  "/api/1.0/users/userExists/{email}":
    get:
      tags:
      - User
      operationId: userEmailExists
      parameters:
      - name: email
        in: path
        description: email
        required: true
        type: string
      responses:
        '200':
          description: Successfully retrieved result!
          schema:
            type: boolean
  "/api/1.0/users/id/{id}":
    get:
      tags:
      - User
      summary: Get specific User by id
      operationId: findUserById
      parameters:
      - name: id
        in: path
        description: id
        required: true
        type: integer
        format: int64
      responses:
        '200':
          description: Successfully retrieved result!
          schema:
            "$ref": "#/definitions/UserDto"
definitions:
  UserDto:
    type: object
    required:
    - id
    - email
    - firstName
    - lastName
    properties:
      id:
        type: integer
        format: int64
      email:
        type: string
      firstName:
        type: string
      lastName:
        type: string
    title: UserDto

Command line used for generation

openapi-generator generate --additional-properties supportsES6=true -i ./openapi.yaml -g typescript-fetch -o out/typescript-fetch

Steps to reproduce

Save yaml to openapi.yaml in current directory and run the command above.

Suggest a fix

Since true and false are both valid json responses, should the return statement simple be return new runtime.JSONApiResponse(response); (This also satisfies the typechecker)