[BUG] InlineModelResolver not ignoring additional properties of $ref fixed field

Description

When the specification has additional properties next to $ref fixed field (in the example below, $ref has an additional property type: object), OpenAPI Generator produces error:

[main] ERROR o.o.codegen.InlineModelResolver - Illegal schema found with $ref combined with other properties, no properties should be defined alongside a $ref:

However, the specification passes validation https://apidevtools.org/swagger-parser/online/ .
So the spec passes Swagger Parser, but fails on OpenAPI Generator.

Thanks to Richard Whitehouse on OpenAPI Generator Slack for pointing me to the OpenAPI spec https://spec.openapis.org/oas/latest.html#fixed-fields-18 which states

This object cannot be extended with additional properties and any properties added SHALL be ignored.

Which indicates that OpenAPI Generator should actually ignore those additional properties instead of resulting in error.

openapi-generator version

Tested on latest master as of 27 Aug 2022 and also published version 6.0.1 .

OpenAPI declaration file content or url

          additionalProperties: false
          properties:
            all_of:
              type: array
              items:
                type: object
                $ref: '#/components/schemas/CatalogsProductGroupFilterKeys'
              minItems: 1

The complete specification is available at https://github.com/pinterest/api-description/blob/main/v5/openapi.yaml .

Generation Details

I have a complete build output on GitHub Actions https://github.com/cliffano/pinterest-sdk/runs/7879342459?check_suite_focus=true

Steps to reproduce

1. curl https://github.com/pinterest/api-description/blob/main/v5/openapi.yaml -o myopenapi.yaml

2. docker
   run
   --rm
   -v pwd:/local openapitools/openapi-generator-cli:v6.0.1
   generate
   --input-spec /local/myopenapi.yaml
   --config /local/clients/ruby/conf.json
   --generator-name ruby
   --output /local/clients/ruby/generated;

Suggest a fix

I think InlineModelResolver conditional check (https://github.com/OpenAPITools/openapi-generator/blob/c135139e3dfba367800a02962f269b57a32ab8d4/modules/openapi-generator/src/main/java/org/openapitools/codegen/InlineModelResolver.java#L224) for additional properties should be removed, allowing the processing to continue.

If this looks correct, I can give it a stab at creating a PR.

Start at modules/openapi-generator/src/main/java/org/openapitools/codegen/InlineModelResolver.java around line 224 and reproduce with the supplied Pinterest OpenAPI file and Docker command. Confirm that a $ref with additional fixed-field properties is processed without the current error and that the generated Ruby output remains valid.