[BUG] [cpp-oatpp-client] Model generator fails with oneOf

Bug Report Checklist

• [ x] Have you provided a full/minimal spec to reproduce the issue?
• [ x] Have you validated the input using an OpenAPI validator?
• [ x] Have you tested with the latest master to confirm the issue still exists?
• [ x] Have you searched for related issues/PRs?

Description

Generating a client using the cpp-oatpp-client generator fails with a ClassCastException if a oneOf is present in the schema.

Exception in thread "main" java.lang.RuntimeException: Could not process model 'Filter_name'.Please make sure that your schema is correct!
 	at org.openapitools.codegen.DefaultGenerator.generateModels(DefaultGenerator.java:519)
 	at org.openapitools.codegen.DefaultGenerator.generateModels(DefaultGenerator.java:444)
 	at org.openapitools.codegen.DefaultGenerator.generate(DefaultGenerator.java:1292)
 	at org.openapitools.codegen.cmd.Generate.execute(Generate.java:535)
 	at org.openapitools.codegen.cmd.OpenApiGeneratorCommand.run(OpenApiGeneratorCommand.java:32)
 	at org.openapitools.codegen.OpenAPIGenerator.main(OpenAPIGenerator.java:66)
Caused by: java.lang.ClassCastException: class io.swagger.v3.oas.models.media.JsonSchema cannot be cast to class io.swagger.v3.oas.models.media.ArraySchema (io.swagger.v3.oas.models.media.JsonSchema and io.swagger.v3.oas.models.media.ArraySchema are in unnamed module of loader 'app')
 	at org.openapitools.codegen.languages.CppOatppClientCodegen.getTypeDeclaration(CppOatppClientCodegen.java:315)
 	at org.openapitools.codegen.DefaultCodegen.fromProperty(DefaultCodegen.java:4184)
 	at org.openapitools.codegen.DefaultCodegen.fromProperty(DefaultCodegen.java:3971)
	at org.openapitools.codegen.languages.AbstractCppCodegen.fromProperty(AbstractCppCodegen.java:275)
 	at org.openapitools.codegen.DefaultCodegen.addVars(DefaultCodegen.java:6181)
 	at org.openapitools.codegen.DefaultCodegen.addVars(DefaultCodegen.java:6102)
 	at org.openapitools.codegen.DefaultCodegen.updateModelForComposedSchema(DefaultCodegen.java:2863)
 	at org.openapitools.codegen.DefaultCodegen.fromModel(DefaultCodegen.java:3153)
 	at org.openapitools.codegen.languages.CppOatppClientCodegen.fromModel(CppOatppClientCodegen.java:182)
 	at org.openapitools.codegen.DefaultGenerator.processModels(DefaultGenerator.java:1754)
 	at org.openapitools.codegen.DefaultGenerator.generateModels(DefaultGenerator.java:514)
 	... 5 more

openapi-generator version

7.21.0-SNAPSHOT

OpenAPI declaration file content or url

  "components" : {
    "schemas" : {
      "Filter" : {
        "type" : "object",
        "properties" : {
          "name" : {
            "oneOf" : [ {
              "$ref" : "#/components/schemas/EqualsPropertyFilter"
            }, {
              "$ref" : "#/components/schemas/EndsWithPropertyFilter"
            }, {
              "$ref" : "#/components/schemas/InPropertyFilter"
            }, {
              "type" : "null"
            } ]
          },
          ....
      },
      "EndsWithPropertyFilter" : {
        "type" : "object",
        "properties" : {
          "endsWith" : {
            "type" : "string"
          }
        },
        "required" : [ "endsWith" ]
      },
      "EqualsPropertyFilter" : {
        "type" : "object",
        "properties" : {
          "eq" : {
            "type" : "string"
          }
        },
        "required" : [ "eq" ]
      },
      "InPropertyFilter" : {
        "type" : "object",
        "properties" : {
          "in" : {
            "type" : "array",
            "items" : {
              "type" : "string"
            }
          }
        },
        "required" : [ "in" ]
      }
    }
  }

Generation Details

openapi-generator-cli generate -i openapi-spec.json -o api-cpp -g cpp-oatpp-client

Suggest a fix

Other C++ generators use a std::variant<Type1, Type2, ...> to represent oneOf, anyOf. This code is missing from the cpp-oatpp-client generator.

See for example https://github.com/OpenAPITools/openapi-generator/blob/a2fd144758a68c107c618eb1393b6f3f02e159bd/modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/CppHttplibServerCodegen.java#L1502

Reproduce the failure with the supplied OpenAPI declaration and the openapi-generator-cli generate -i openapi-spec.json -o api-cpp -g cpp-oatpp-client command. Start at CppOatppClientCodegen.java:315 and :182, then compare the oneOf/anyOf handling in CppHttplibServerCodegen.java:1502. Done means generation succeeds for the provided schema without the ClassCastException.