[BUG][GO] Invalid property type for free form object

[shaxbee]

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?map[string]interface{}

Description

Free form object property yields map[string]map[string]interface{} where it should be map[string]interface{}

openapi-generator version

4.0.1

OpenAPI declaration file content or url

components:
  schemas:
    Response:
      type: object
      properties:
        payload:
          type: object
          additionalProperties: {}

Command line used for generation

openapi-generator-cli generate -i ../api.yaml -g go -o . -DpackageName=api -DwithGoCodegenComment

Steps to reproduce

Generate go client using given schema.

Suggest a fix

Tried changing getTypeDeclaration in AbstractGoCodegen.java to use ModelUtils.isFreeFormObject to no avail.
Also attempted changing template with no success.

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[grokify]

I've run into a similar issue where:

type: object results in a map[string]string when it should be a map[string]interface{}

[docwhat]

I was able to work around OP's issue by passing this flag to openapi-generator-cli:

--type-mappings=object=interface{}

(I would have thought I would have to pass type[string]interface{} but I guess not?)