Use the model where the interface represents the one-of 'root' and the options implement that interface.

Author: koenlavooij

modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/KotlinServerCodegen.java

@@ -106,6 +106,9 @@ public class KotlinServerCodegen extends AbstractKotlinCodegen implements BeanVa
     public KotlinServerCodegen() {
         super();
 
+        useOneOfInterfaces = true;
+        addOneOfInterfaceImports = true;
+
         modifyFeatureSet(features -> features
                 .includeDocumentationFeatures(DocumentationFeature.Readme)
                 .wireFormatFeatures(EnumSet.of(WireFormatFeature.JSON, WireFormatFeature.XML))

modules/openapi-generator/src/main/resources/kotlin-server/data_class.mustache

@@ -28,7 +28,7 @@ import java.io.Serializable
 {{/-last}}{{/requiredVars}}{{#hasRequired}}{{#hasOptional}},
 {{/hasOptional}}{{/hasRequired}}{{#optionalVars}}{{>data_class_opt_var}}{{^-last}},
 {{/-last}}{{/optionalVars}}
-){{^isKtor}}{{^serializableModel}}{{#parcelizeModels}} : Parcelable{{/parcelizeModels}}{{/serializableModel}}{{^parcelizeModels}}{{#serializableModel}}: Serializable {{/serializableModel}}{{/parcelizeModels}}{{#parcelizeModels}}{{#serializableModel}} : Parcelable, Serializable {{/serializableModel}}{{/parcelizeModels}}{{/isKtor}}
+){{^isKtor}}{{#vendorExtensions.x-implements}}: {{{.}}}{{/vendorExtensions.x-implements}}{{^serializableModel}}{{#parcelizeModels}} : Parcelable{{/parcelizeModels}}{{/serializableModel}}{{^parcelizeModels}}{{#serializableModel}}: Serializable {{/serializableModel}}{{/parcelizeModels}}{{#parcelizeModels}}{{#serializableModel}} : Parcelable, Serializable {{/serializableModel}}{{/parcelizeModels}}{{/isKtor}}
 {{#vendorExtensions.x-has-data-class-body}}
 {
 {{/vendorExtensions.x-has-data-class-body}}

modules/openapi-generator/src/main/resources/kotlin-server/model.mustache

@@ -6,6 +6,6 @@ package {{modelPackage}}
 
 {{#models}}
 {{#model}}
-{{#isEnum}}{{>enum_class}}{{/isEnum}}{{^isEnum}}{{>data_class}}{{/isEnum}}
+{{#isEnum}}{{>enum_class}}{{/isEnum}}{{^isEnum}}{{#oneOf}}{{>oneof_model}}{{/oneOf}}{{^oneOf}}{{>data_class}}{{/oneOf}}{{/isEnum}}
 {{/model}}
 {{/models}}

modules/openapi-generator/src/main/resources/kotlin-server/oneof_model.mustache

@@ -0,0 +1,3 @@
+{{#-first}}
+interface {{classname}}
+{{/-first}}