[kotlin] fix: enumUnknownDefaultCase's moshi fallback adapters break nullable enum fields (#24894)

* [kotlin] fix: make the enumUnknownDefaultCase moshi fallback adapters null-safe

SerializerHelper registers a moshi EnumJsonAdapter with withUnknownFallback
per enum, but EnumJsonAdapter is not null-safe and it is registered bare:
any model with a nullable enum property throws "value was null! Wrap in
.nullSafe() to write nullable values" on a null value, reading and writing
alike. The flag traded unknown-value tolerance for a regression on every
optional enum field. Append .nullSafe() in both branches.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01GcwZ1arjLZNpetHz2a3TJz

* test: [kotlin] shorten the null-safe adapter test comment

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

---------

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

Author: wiebren

modules/openapi-generator/src/main/resources/kotlin-client/jvm-common/infrastructure/SerializerHelper.kt.mustache

@@ -19,7 +19,7 @@ import com.squareup.moshi.adapters.EnumJsonAdapter
 {{#enumVars}}
 {{#-last}}
             .add({{modelPackage}}.{{classname}}::class.java, EnumJsonAdapter.create({{modelPackage}}.{{classname}}::class.java)
-                .withUnknownFallback({{modelPackage}}.{{classname}}.{{&name}}))
+                .withUnknownFallback({{modelPackage}}.{{classname}}.{{&name}}).nullSafe())
 {{/-last}}
 {{/enumVars}}
 {{/allowableValues}}
@@ -33,7 +33,7 @@ import com.squareup.moshi.adapters.EnumJsonAdapter
 {{#enumVars}}
 {{#-last}}
             .add({{modelPackage}}.{{classname}}.{{{nameInPascalCase}}}::class.java, EnumJsonAdapter.create({{modelPackage}}.{{classname}}.{{{nameInPascalCase}}}::class.java)
-                .withUnknownFallback({{modelPackage}}.{{classname}}.{{{nameInPascalCase}}}.{{&name}}))
+                .withUnknownFallback({{modelPackage}}.{{classname}}.{{{nameInPascalCase}}}.{{&name}}).nullSafe())
 {{/-last}}
 {{/enumVars}}
 {{/allowableValues}}

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

@@ -990,6 +990,33 @@ public void testJacksonEnumsExposeDecodeOrNullHelper() throws IOException {
               "Expected exactly one @JsonCreator annotation in the generated enum, found " + jsonCreatorCount);
   }
 
+  @Test
+  public void testMoshiEnumUnknownDefaultCaseAdaptersAreNullSafe() throws IOException {
+      File output = Files.createTempDirectory("test").toFile();
+      output.deleteOnExit();
+
+      final CodegenConfigurator configurator = new CodegenConfigurator()
+              .setGeneratorName(KOTLIN_GENERATOR)
+              .setLibrary("jvm-okhttp4")
+              .setAdditionalProperties(new HashMap<>() {{
+                put(CodegenConstants.SERIALIZATION_LIBRARY, "moshi");
+                put(CodegenConstants.ENUM_UNKNOWN_DEFAULT_CASE, "true");
+              }})
+              .setInputSpec("src/test/resources/3_0/enum.yaml")
+              .setOutputDir(output.getAbsolutePath().replace("\\", "/"));
+
+      final ClientOptInput clientOptInput = configurator.toClientOptInput();
+      DefaultGenerator generator = new DefaultGenerator();
+
+      generator.opts(clientOptInput).generate();
+
+      final Path helperKt = Paths.get(output + "/src/main/kotlin/org/openapitools/client/infrastructure/SerializerHelper.kt");
+
+      // EnumJsonAdapter is not null-safe, so every registered fallback adapter must be wrapped
+      TestUtils.assertFileContains(helperKt, ".nullSafe())");
+      TestUtils.assertFileNotContains(helperKt, "unknown_default_open_api))");
+  }
+
   @Test
   public void testJacksonEnumsWithUnknownDefaultCase() throws IOException {
       File output = Files.createTempDirectory("test").toFile();

samples/client/petstore/kotlin-enum-default-value/src/main/kotlin/org/openapitools/client/infrastructure/SerializerHelper.kt

@@ -7,14 +7,14 @@ object SerializerHelper {
     fun addEnumUnknownDefaultCase(moshiBuilder: Moshi.Builder): Moshi.Builder {
         return moshiBuilder
             .add(org.openapitools.client.models.ComplexEnum::class.java, EnumJsonAdapter.create(org.openapitools.client.models.ComplexEnum::class.java)
-                .withUnknownFallback(org.openapitools.client.models.ComplexEnum.unknown_default_open_api))
+                .withUnknownFallback(org.openapitools.client.models.ComplexEnum.unknown_default_open_api).nullSafe())
             .add(org.openapitools.client.models.ModelWithEnumPropertyHavingDefault.PropertyName::class.java, EnumJsonAdapter.create(org.openapitools.client.models.ModelWithEnumPropertyHavingDefault.PropertyName::class.java)
-                .withUnknownFallback(org.openapitools.client.models.ModelWithEnumPropertyHavingDefault.PropertyName.unknown_default_open_api))
+                .withUnknownFallback(org.openapitools.client.models.ModelWithEnumPropertyHavingDefault.PropertyName.unknown_default_open_api).nullSafe())
             .add(org.openapitools.client.models.PropertyOfDay.DaysOfWeek::class.java, EnumJsonAdapter.create(org.openapitools.client.models.PropertyOfDay.DaysOfWeek::class.java)
-                .withUnknownFallback(org.openapitools.client.models.PropertyOfDay.DaysOfWeek.unknown_default_open_api))
+                .withUnknownFallback(org.openapitools.client.models.PropertyOfDay.DaysOfWeek.unknown_default_open_api).nullSafe())
             .add(org.openapitools.client.models.PropertyOfDay.MonthOfYear::class.java, EnumJsonAdapter.create(org.openapitools.client.models.PropertyOfDay.MonthOfYear::class.java)
-                .withUnknownFallback(org.openapitools.client.models.PropertyOfDay.MonthOfYear.unknown_default_open_api))
+                .withUnknownFallback(org.openapitools.client.models.PropertyOfDay.MonthOfYear.unknown_default_open_api).nullSafe())
             .add(org.openapitools.client.models.PropertyOfDay.HolidayTypes::class.java, EnumJsonAdapter.create(org.openapitools.client.models.PropertyOfDay.HolidayTypes::class.java)
-                .withUnknownFallback(org.openapitools.client.models.PropertyOfDay.HolidayTypes.unknown_default_open_api))
+                .withUnknownFallback(org.openapitools.client.models.PropertyOfDay.HolidayTypes.unknown_default_open_api).nullSafe())
     }
 }