[kotlin][client] Register enumUnknownDefaultCase as a CliOption so it appears in config-help and the docs

docs/generators/kotlin.md

@@ -25,6 +25,7 @@ These options may be applied as additional-properties (cli) or configOptions (pl
 |companionObject|Whether to generate companion objects in data classes, enabling companion extensions.| |false|
 |dateLibrary|Option. Date library to use|<dl><dt>**threetenbp-localdatetime**</dt><dd>Threetenbp - Backport of JSR310 (jvm only, for legacy app only)</dd><dt>**kotlinx-datetime**</dt><dd>kotlinx-datetime (preferred for multiplatform)</dd><dt>**string**</dt><dd>String</dd><dt>**java8-localdatetime**</dt><dd>Java 8 native JSR310 (jvm only, for legacy app only)</dd><dt>**java8**</dt><dd>Java 8 native JSR310 (jvm only, preferred for jdk 1.8+)</dd><dt>**threetenbp**</dt><dd>Threetenbp - Backport of JSR310 (jvm only, preferred for jdk &lt; 1.8)</dd></dl>|java8|
 |enumPropertyNaming|Naming convention for enum properties: 'camelCase', 'PascalCase', 'snake_case', 'UPPERCASE', 'original', and 'bestEffortBacktick' (like 'original' but tries to wrap values in backticks before falling back to sanitizing, e.g. `name,asc` stays `name,asc` rather than becoming nameCommaAsc; useful for sort/order enums)| |original|
+|enumUnknownDefaultCase|Add an `unknown_default_open_api` enum case as a fallback for unrecognized values. Only `moshi`(serializationLibrary) decodes every unknown value to it: `jackson` skips nullable enums, `kotlinx_serialization` skips non-string enums, and neither `gson`(serializationLibrary) nor `multiplatform`(library) decodes to it at all.|<dl><dt>**false**</dt><dd>No changes to the enums are made, this is the default option.</dd><dt>**true**</dt><dd>Each enum gains an `unknown_default_open_api` case.</dd></dl>|false|
 |explicitApi|Generates code with explicit access modifiers to comply with Kotlin Explicit API Mode.| |false|
 |failOnUnknownProperties|Fail Jackson de-serialization on unknown properties| |false|
 |generateOneOfAnyOfWrappers|Generate oneOf, anyOf schemas as wrappers. Only `jvm-retrofit2`(library) with `gson` or `kotlinx_serialization`(serializationLibrary) support this option.| |false|

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

@@ -316,6 +316,19 @@ public KotlinClientCodegen() {
         cliOptions.add(CliOption.newBoolean(USE_RESPONSE_AS_RETURN_TYPE, "When using retrofit2 and coroutines, use `Response`<`T`> as return type instead of `T`.", true));
 
         cliOptions.add(CliOption.newBoolean(USE_JACKSON_3, "Use Jackson 3 dependencies (tools.jackson package). Requires serializationLibrary=jackson. Incompatible with openApiNullable."));
+
+        // AbstractKotlinCodegen calls cliOptions.clear(), dropping DefaultCodegen's registration.
+        // Re-registered on kotlin alone: no other Kotlin generator's templates implement the fallback.
+        CliOption enumUnknownDefaultCaseOpt = CliOption.newBoolean(
+                CodegenConstants.ENUM_UNKNOWN_DEFAULT_CASE,
+                "Add an `unknown_default_open_api` enum case as a fallback for unrecognized values. Only `moshi`(serializationLibrary) decodes every unknown value to it: `jackson` skips nullable enums, `kotlinx_serialization` skips non-string enums, and neither `gson`(serializationLibrary) nor `multiplatform`(library) decodes to it at all.");
+        Map<String, String> enumUnknownDefaultCaseOpts = new HashMap<>();
+        enumUnknownDefaultCaseOpts.put("false",
+                "No changes to the enums are made, this is the default option.");
+        enumUnknownDefaultCaseOpts.put("true",
+                "Each enum gains an `unknown_default_open_api` case.");
+        enumUnknownDefaultCaseOpt.setEnum(enumUnknownDefaultCaseOpts);
+        cliOptions.add(enumUnknownDefaultCaseOpt);
     }
 
     @Override

modules/openapi-generator/src/test/java/org/openapitools/codegen/kotlin/KotlinClientCodegenModelTest.java

@@ -42,6 +42,7 @@
 import java.util.HashMap;
 import java.util.List;
 import java.util.Map;
+import java.util.Set;
 
 import static org.openapitools.codegen.CodegenConstants.*;
 import static org.openapitools.codegen.languages.KotlinClientCodegen.*;
@@ -1505,4 +1506,21 @@ public void testNoDefaultImplWhenNeitherSourceIsSet() throws IOException {
         Assert.assertTrue(sawJsonTypeInfo,
                 "Expected at least one generated model with @JsonTypeInfo to exercise the code path");
     }
+
+    /**
+     * AbstractKotlinCodegen calls cliOptions.clear(), so an option inherited from DefaultCodegen stays
+     * functional while vanishing from config-help and docs/generators/kotlin.md. That is how
+     * enumUnknownDefaultCase went undocumented for years; this guards the re-registration.
+     */
+    @Test
+    public void testEnumUnknownDefaultCaseIsRegisteredAsCliOption() {
+        CliOption option = new KotlinClientCodegen().cliOptions().stream()
+                .filter(o -> CodegenConstants.ENUM_UNKNOWN_DEFAULT_CASE.equals(o.getOpt()))
+                .findFirst()
+                .orElse(null);
+
+        Assert.assertNotNull(option, CodegenConstants.ENUM_UNKNOWN_DEFAULT_CASE + " is not registered");
+        Assert.assertEquals(option.getDefault(), "false");
+        Assert.assertEquals(option.getEnum().keySet(), Set.of("true", "false"));
+    }
 }