Support Kotlin synthetic classes in MethodParameter and SpEL

Closes gh-23812

spring-core/src/main/java/org/springframework/core/MethodParameter.java

@@ -908,9 +908,14 @@ public class MethodParameter {
 		 * functions via Kotlin reflection.
 		 */
 		static private Type getGenericReturnType(Method method) {
-			KFunction<?> function = ReflectJvmMapping.getKotlinFunction(method);
-			if (function != null && function.isSuspend()) {
-				return ReflectJvmMapping.getJavaType(function.getReturnType());
+			try {
+				KFunction<?> function = ReflectJvmMapping.getKotlinFunction(method);
+				if (function != null && function.isSuspend()) {
+					return ReflectJvmMapping.getJavaType(function.getReturnType());
+				}
+			}
+			catch (UnsupportedOperationException ex) {
+				// probably a synthetic class - let's use java reflection instead
 			}
 			return method.getGenericReturnType();
 		}
@@ -920,10 +925,15 @@ public class MethodParameter {
 		 * functions via Kotlin reflection.
 		 */
 		static private Class<?> getReturnType(Method method) {
-			KFunction<?> function = ReflectJvmMapping.getKotlinFunction(method);
-			if (function != null && function.isSuspend()) {
-				Type paramType = ReflectJvmMapping.getJavaType(function.getReturnType());
-				return ResolvableType.forType(paramType).resolve(method.getReturnType());
+			try {
+				KFunction<?> function = ReflectJvmMapping.getKotlinFunction(method);
+				if (function != null && function.isSuspend()) {
+					Type paramType = ReflectJvmMapping.getJavaType(function.getReturnType());
+					return ResolvableType.forType(paramType).resolve(method.getReturnType());
+				}
+			}
+			catch (UnsupportedOperationException ex) {
+				// probably a synthetic class - let's use java reflection instead
 			}
 			return method.getReturnType();
 		}

spring-expression/spring-expression.gradle

@@ -1,5 +1,9 @@
 description = "Spring Expression Language (SpEL)"
 
+apply plugin: "kotlin"
+
 dependencies {
 	compile(project(":spring-core"))
+	testCompile("org.jetbrains.kotlin:kotlin-reflect")
+	testCompile("org.jetbrains.kotlin:kotlin-stdlib")
 }

spring-expression/src/test/kotlin/org/springframework/expression/spel/KotlinSpelReproTests.kt

@@ -0,0 +1,28 @@
+package org.springframework.expression.spel
+
+import org.assertj.core.api.Assertions.assertThat
+import org.junit.jupiter.api.Test
+import org.springframework.expression.ExpressionParser
+import org.springframework.expression.spel.standard.SpelExpressionParser
+
+class KotlinSpelReproTests {
+
+	private val parser: ExpressionParser = SpelExpressionParser()
+
+	private val context = TestScenarioCreator.getTestEvaluationContext()
+
+
+	@Test
+	fun `gh-23812 SpEL cannot invoke Kotlin synthetic classes`() {
+		val expr = parser.parseExpression("new org.springframework.expression.spel.KotlinSpelReproTests\$Config().kotlinSupplier().invoke()")
+		assertThat(expr.getValue(context)).isEqualTo("test")
+	}
+
+	class Config {
+
+		fun kotlinSupplier(): () -> String {
+			return { "test" }
+		}
+
+	}
+}