[kotlin] Serialize inline number enums by value with kotlinx_serialization

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

@@ -43,6 +43,12 @@ import kotlinx.serialization.encoding.Decoder
 import kotlinx.serialization.encoding.Encoder
 {{/enumUnknownDefaultCase}}
 {{#hasEnums}}
+{{^enumUnknownDefaultCase}}
+import kotlinx.serialization.KSerializer
+import kotlinx.serialization.builtins.serializer
+import kotlinx.serialization.encoding.Decoder
+import kotlinx.serialization.encoding.Encoder
+{{/enumUnknownDefaultCase}}
 {{/hasEnums}}
 {{#discriminator}}
 import kotlinx.serialization.ExperimentalSerializationApi
@@ -144,7 +150,7 @@ import {{packageName}}.infrastructure.ITransformForStorage
      */
     {{^multiplatform}}
     {{#kotlinx_serialization}}
-    {{#serializableModel}}@KSerializable{{/serializableModel}}{{^serializableModel}}@Serializable{{#enumUnknownDefaultCase}}(with = {{{nameInPascalCase}}}Serializer::class){{/enumUnknownDefaultCase}}{{/serializableModel}}
+    {{#serializableModel}}@KSerializable{{/serializableModel}}{{^serializableModel}}@Serializable{{#enumUnknownDefaultCase}}(with = {{{nameInPascalCase}}}Serializer::class){{/enumUnknownDefaultCase}}{{/serializableModel}}{{^enumUnknownDefaultCase}}{{^isString}}{{^isNumber}}{{^isContainer}}(with = {{{nameInPascalCase}}}Serializer::class){{/isContainer}}{{/isNumber}}{{/isString}}{{/enumUnknownDefaultCase}}
     {{/kotlinx_serialization}}
     {{#moshi}}
     @JsonClass(generateAdapter = false)
@@ -201,6 +207,30 @@ import {{packageName}}.infrastructure.ITransformForStorage
         }
     }
     {{/enumUnknownDefaultCase}}
+    {{^multiplatform}}
+    {{^enumUnknownDefaultCase}}
+    {{^isString}}
+    {{^isNumber}}
+    {{^isContainer}}
+
+    internal object {{nameInPascalCase}}Serializer : KSerializer<{{nameInPascalCase}}> {
+        override val descriptor = {{dataType}}.serializer().descriptor
+
+        override fun deserialize(decoder: Decoder): {{nameInPascalCase}} {
+            val value = decoder.decodeSerializableValue({{dataType}}.serializer())
+            return {{nameInPascalCase}}.entries.firstOrNull { it.value == value }
+                ?: throw IllegalArgumentException("Unknown enum value: $value")
+        }
+
+        override fun serialize(encoder: Encoder, value: {{nameInPascalCase}}) {
+            encoder.encodeSerializableValue({{dataType}}.serializer(), value.value)
+        }
+    }
+    {{/isContainer}}
+    {{/isNumber}}
+    {{/isString}}
+    {{/enumUnknownDefaultCase}}
+    {{/multiplatform}}
     {{/kotlinx_serialization}}
 {{/isEnum}}
 {{/vars}}

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

@@ -1582,4 +1582,68 @@ public void testEnumUnknownDefaultCaseIsRegisteredAsCliOption() {
         Assert.assertEquals(option.getDefault(), "false");
         Assert.assertEquals(option.getEnum().keySet(), Set.of("true", "false"));
     }
+
+    private Path generateKotlinxInlineNumericEnumModels(boolean serializableModel) throws IOException {
+        File output = Files.createTempDirectory("test").toFile();
+        output.deleteOnExit();
+
+        final CodegenConfigurator configurator = new CodegenConfigurator()
+                .setGeneratorName(KOTLIN_GENERATOR)
+                .setInputSpec("src/test/resources/3_0/kotlin/issue25459-kotlinx-inline-numeric-enum.yaml")
+                .setOutputDir(output.getAbsolutePath().replace("\\", "/"))
+                .addAdditionalProperty(CodegenConstants.SERIALIZATION_LIBRARY, "kotlinx_serialization")
+                .addAdditionalProperty(CodegenConstants.SERIALIZABLE_MODEL, serializableModel);
+
+        DefaultGenerator generator = new DefaultGenerator();
+        generator.opts(configurator.toClientOptInput()).generate();
+
+        return Paths.get(output.getAbsolutePath(), "src/main/kotlin/org/openapitools/client/models");
+    }
+
+    @Test
+    public void testKotlinxSerializationInlineNumericEnumUsesValueSerializer() throws IOException {
+        Path models = generateKotlinxInlineNumericEnumModels(false);
+        Path recordingKt = models.resolve("Recording.kt");
+
+        // A plain @Serializable enum is written by serial name, so Plays would be the JSON string "1"
+        // instead of the number 1. The value serializer reads and writes the Int itself.
+        TestUtils.assertFileContains(recordingKt,
+                "@Serializable(with = PlaysSerializer::class)",
+                "enum class Plays(val value: kotlin.Int)",
+                "internal object PlaysSerializer : KSerializer<Plays>",
+                "decoder.decodeSerializableValue(kotlin.Int.serializer())",
+                "encoder.encodeSerializableValue(kotlin.Int.serializer(), value.value)",
+                "import kotlinx.serialization.KSerializer",
+                "import kotlinx.serialization.builtins.serializer",
+                "import kotlinx.serialization.encoding.Decoder",
+                "import kotlinx.serialization.encoding.Encoder");
+        // String enums stay on their @SerialName values.
+        TestUtils.assertFileNotContains(recordingKt, "ModeSerializer");
+        // Models without enums must not pick up the serializer imports.
+        TestUtils.assertFileNotContains(models.resolve("Label.kt"), "KSerializer");
+    }
+
+    @Test
+    public void testKotlinxSerializationInlineBigDecimalEnumKeepsPlainSerializable() throws IOException {
+        Path recordingKt = generateKotlinxInlineNumericEnumModels(false).resolve("Recording.kt");
+
+        // type: number maps to java.math.BigDecimal, which has no serializer() to build a value serializer from.
+        TestUtils.assertFileContains(recordingKt,
+                "@Serializable enum class Ratio(val value: java.math.BigDecimal)");
+        TestUtils.assertFileNotContains(recordingKt, "RatioSerializer", "java.math.BigDecimal.serializer()");
+        // type: number with format double maps to kotlin.Double and keeps the value serializer.
+        TestUtils.assertFileContains(recordingKt,
+                "@Serializable(with = WeightSerializer::class)",
+                "encoder.encodeSerializableValue(kotlin.Double.serializer(), value.value)");
+    }
+
+    @Test
+    public void testKotlinxSerializationInlineNumericEnumUsesValueSerializerForSerializableModel() throws IOException {
+        // serializableModel imports kotlinx's Serializable under the alias KSerializable.
+        Path recordingKt = generateKotlinxInlineNumericEnumModels(true).resolve("Recording.kt");
+
+        TestUtils.assertFileContains(recordingKt,
+                "@KSerializable(with = PlaysSerializer::class)",
+                "internal object PlaysSerializer : KSerializer<Plays>");
+    }
 }

modules/openapi-generator/src/test/resources/3_0/kotlin/issue25459-kotlinx-inline-numeric-enum.yaml

@@ -0,0 +1,46 @@
+openapi: 3.0.3
+info:
+  title: 'Issue 25459 kotlinx_serialization inline numeric enum'
+  version: latest
+paths:
+  '/recording':
+    get:
+      operationId: getRecording
+      responses:
+        '200':
+          description: Success
+          content:
+            application/json:
+              schema:
+                $ref: '#/components/schemas/Recording'
+components:
+  schemas:
+    Recording:
+      type: object
+      required: [plays, mode, speed]
+      properties:
+        plays:
+          type: integer
+          enum: [1, 2]
+        mode:
+          type: string
+          enum: [fast, slow]
+        ratio:
+          type: number
+          enum: [0.5, 1.5]
+        weight:
+          type: number
+          format: double
+          enum: [0.25, 0.75]
+        speed:
+          $ref: '#/components/schemas/Speed'
+        label:
+          $ref: '#/components/schemas/Label'
+    Speed:
+      type: integer
+      enum: [1, 2]
+    Label:
+      type: object
+      properties:
+        text:
+          type: string

samples/client/petstore/kotlin-jvm-ktor-kotlinx_serialization/src/main/kotlin/org/openapitools/client/models/Order.kt

@@ -27,6 +27,10 @@ package org.openapitools.client.models
 import kotlinx.serialization.Serializable
 import kotlinx.serialization.SerialName
 import kotlinx.serialization.Contextual
+import kotlinx.serialization.KSerializer
+import kotlinx.serialization.builtins.serializer
+import kotlinx.serialization.encoding.Decoder
+import kotlinx.serialization.encoding.Encoder
 
 /**
  * An order for a pets from the pet store

samples/client/petstore/kotlin-jvm-ktor-kotlinx_serialization/src/main/kotlin/org/openapitools/client/models/Pet.kt

@@ -29,6 +29,10 @@ import org.openapitools.client.models.Tag
 import kotlinx.serialization.Serializable
 import kotlinx.serialization.SerialName
 import kotlinx.serialization.Contextual
+import kotlinx.serialization.KSerializer
+import kotlinx.serialization.builtins.serializer
+import kotlinx.serialization.encoding.Decoder
+import kotlinx.serialization.encoding.Encoder
 
 /**
  * A pet for sale in the pet store

samples/client/petstore/kotlin-retrofit2-kotlinx_serialization/src/main/kotlin/org/openapitools/client/models/Order.kt

@@ -27,6 +27,10 @@ package org.openapitools.client.models
 import kotlinx.serialization.Serializable as KSerializable
 import kotlinx.serialization.SerialName
 import kotlinx.serialization.Contextual
+import kotlinx.serialization.KSerializer
+import kotlinx.serialization.builtins.serializer
+import kotlinx.serialization.encoding.Decoder
+import kotlinx.serialization.encoding.Encoder
 import java.io.Serializable
 
 /**

samples/client/petstore/kotlin-retrofit2-kotlinx_serialization/src/main/kotlin/org/openapitools/client/models/Pet.kt

@@ -29,6 +29,10 @@ import org.openapitools.client.models.Tag
 import kotlinx.serialization.Serializable as KSerializable
 import kotlinx.serialization.SerialName
 import kotlinx.serialization.Contextual
+import kotlinx.serialization.KSerializer
+import kotlinx.serialization.builtins.serializer
+import kotlinx.serialization.encoding.Decoder
+import kotlinx.serialization.encoding.Encoder
 import java.io.Serializable
 
 /**