[BUG] wrong interpretation of missing type in anyOf/allOf/oneOf as nullable

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?
• [Optional] Sponsorship to speed up the bug fix or feature request (example)

Description

Since PR #15698, the generator assumes that a schema object without a type property means that the object is nullable. That assumption is as far as I can see not support by either OAS 3.0.3, OAS 3.1 or JSON Schema.

For OAS 3.0.3 the situation is explicitly documented at https://github.com/OAI/OpenAPI-Specification/blob/main/versions/3.0.3.md#data-types, which states:

null is not supported as a type

For OAS 3.1 the null type is supported, however there is nothing in OAS nor JSON Schema that mandates that a missing type equals the null type.

openapi-generator version

• 7.0.0
• from master branch

OpenAPI declaration file content or url

A OpenAPI specification as input:

openapi: 3.0.1
info:
  version: 1.0.0
  title: Example
  license:
    name: MIT
servers:
  - url: http://api.example.xyz/v1
paths:
  /foo:
    get:
      operationId: list
      responses:
        '200':
          description: OK
          content:
            application/json:
              schema:
                $ref: "#/components/schemas/AnyOfRequiredInObject"
components:
  schemas:
    AnyOfRequiredInObject:
      description: to test oneOf with required
      type: object
      oneOf:
        - required: [ field3 ]
        - required: [ field4 ]
      properties:
        field3:
          type: integer
        field4:
          type: integer

is translated in openapi generator to:

    "schemas" : {
      "AnyOfRequiredInObject" : {
        "description" : "to test oneOf with required",
        "nullable" : true,
        "oneOf" : [ ],
        "properties" : {
          "field3" : {
            "type" : "integer"
          },
          "field4" : {
            "type" : "integer"
          }
        },
        "type" : "object"
      }
    }

Note the missing declarations in oneOf and the erroneously added "nullable": true

Generation Details

java -jar modules/openapi-generator-cli/target/openapi-generator-cli.jar generate -g openapi -i test.yaml -o out

Steps to reproduce

run the openapi generator and check the output for missing oneOf and wrong nullable

Related issues/PRs

• #15698
• https://github.com/OAI/OpenAPI-Specification/issues/3148 is somewhat related

Suggest a fix

https://github.com/karzang/openapi-generator/blob/master/modules/openapi-generator/src/main/java/org/openapitools/codegen/OpenAPINormalizer.java#L678-L696 should be changed to something like e.g.:

    private boolean isNullTypeSchema(Schema schema) {
        if (schema == null) {
            return false;
        }

        if ((schema.getType() == null) {
            return false;
        }

        if ((schema.getType().equals("null")) && schema.get$ref() == null) {
            return true;
        }

        // convert referenced enum of null only to `nullable:true`
        Schema referencedSchema = ModelUtils.getReferencedSchema(openAPI, schema);
        if (referencedSchema.getEnum() != null && referencedSchema.getEnum().size() == 1) {
            if ("null".equals(String.valueOf(referencedSchema.getEnum().get(0)))) {
                return true;
            }
        }

        return false;
    }

Start with modules/openapi-generator/src/main/java/org/openapitools/codegen/OpenAPINormalizer.java around lines 678-696, then reproduce the issue with the provided OpenAPI YAML and the openapi generator command. Check how schemas without a type are handled in anyOf, allOf, and oneOf. Done means the oneOf declarations are retained and nullable is not added when type is missing.