Add missing reflection hints on Kotlin classes

This commit adds the relevant reflection hints required by
`KotlinDetector`; this class is trying to detect both `kotlin.Metadata`
and `kotlin.reflect.full.KClasses`.

Fixes gh- 31269

spring-core/src/main/java/org/springframework/aot/hint/support/KotlinDetectorRuntimeHints.java

@@ -0,0 +1,36 @@
+/*
+ * Copyright 2002-2023 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.aot.hint.support;
+
+import org.springframework.aot.hint.RuntimeHints;
+import org.springframework.aot.hint.RuntimeHintsRegistrar;
+import org.springframework.aot.hint.TypeReference;
+
+/**
+ * {@link RuntimeHintsRegistrar} to register hints for {@link org.springframework.core.KotlinDetector}.
+ *
+ * @author Brian Clozel
+ * @since 6.1
+ */
+class KotlinDetectorRuntimeHints  implements RuntimeHintsRegistrar {
+
+	@Override
+	public void registerHints(RuntimeHints hints, ClassLoader classLoader) {
+		hints.reflection().registerType(TypeReference.of("kotlin.Metadata"))
+				.registerType(TypeReference.of("kotlin.reflect.full.KClasses"));
+	}
+}

spring-core/src/main/resources/META-INF/spring/aot.factories

@@ -1,3 +1,4 @@
 org.springframework.aot.hint.RuntimeHintsRegistrar=\
+org.springframework.aot.hint.support.KotlinDetectorRuntimeHints,\
 org.springframework.aot.hint.support.ObjectToObjectConverterRuntimeHints,\
 org.springframework.aot.hint.support.SpringFactoriesLoaderRuntimeHints

spring-core/src/test/java/org/springframework/aot/hint/support/KotlinDetectorRuntimeHintsTests.java

@@ -0,0 +1,56 @@
+/*
+ * Copyright 2002-2023 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.aot.hint.support;
+
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+
+import org.springframework.aot.hint.RuntimeHints;
+import org.springframework.aot.hint.RuntimeHintsRegistrar;
+import org.springframework.aot.hint.predicate.RuntimeHintsPredicates;
+import org.springframework.core.io.support.SpringFactoriesLoader;
+import org.springframework.util.ClassUtils;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+/**
+ * Tests for {@link KotlinDetectorRuntimeHints}.
+ * @author Brian Clozel
+ */
+class KotlinDetectorRuntimeHintsTests {
+
+	private RuntimeHints hints;
+
+	@BeforeEach
+	void setup() {
+		this.hints = new RuntimeHints();
+		SpringFactoriesLoader.forResourceLocation("META-INF/spring/aot.factories")
+				.load(RuntimeHintsRegistrar.class).forEach(registrar -> registrar
+						.registerHints(this.hints, ClassUtils.getDefaultClassLoader()));
+	}
+
+	@Test
+	void kotlinMetadataHasHints() {
+		assertThat(RuntimeHintsPredicates.reflection().onType(kotlin.Metadata.class)).accepts(this.hints);
+	}
+
+	@Test
+	void kotlinReflectHasHints() {
+		assertThat(RuntimeHintsPredicates.reflection().onType(kotlin.reflect.full.KClasses.class)).accepts(this.hints);
+	}
+
+}