[BUG] [typescript-fetch] `Module '"./MyEnum"' has no exported member 'instanceOfMyEnum'.` and `Spread types may only be created from object types.`

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?
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Description

My CLI:

yarn g:openapi-generator-cli generate -i ./api-docs.yaml -g typescript-fetch -o ./src --additional-properties=supportsES6=true,disallowAdditionalPropertiesIfNotPresent=false

My Schema:

components:
  schemas:
    MyEnum:
      type: string
      enum:
      - ONE
      - TWO
      - THREE
      - FOUR
      - FIVE
      - ETC
    MyObject:
      type: object
      properties:
        type:
          nullable: true
          oneOf:
          - $ref: '#/components/schemas/MyEnum'

Error that happens because certain files are expecting to use instanceOfMyEnum - which does not exist:

models/MyModel.ts:17:5 - error TS2305: Module '"./MyEnum"' has no exported member 'instanceOfMyEnum'.

17     instanceOfMyEnum,
       ~~~~~~~~~~~~~~~~~~~~~~~

Error that happens because certain files are expecting to spread the result of MyEnumFromJSONTyped(json, true):

models/MyModel.ts:38:14 - error TS2698: Spread types may only be created from object types.

38     return { ...MyEnumFromJSONTyped(json, true) };
                ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

openapi-generator version

TS2305 errors occur on: >=6.1.0
TS2698 errors occur on >=6.0.0

Neither occur on 5.4.0.

OpenAPI declaration file content or url

Generation Details

Steps to reproduce

Related issues/PRs

• https://github.com/OpenAPITools/openapi-generator/issues/14611
• https://github.com/OpenAPITools/openapi-generator/issues/14763

Suggest a fix

Reproduce the issue with the provided CLI command and OpenAPI schema, then inspect the generated models/MyModel.ts and MyEnum.ts files for the missing export and invalid spread. Compare the output with openapi-generator 5.4.0 and later versions; done means the generated TypeScript compiles without TS2305 or TS2698 errors.