Support Kotlin synthetic classes in MethodParameter and SpEL

Closes gh-23812

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" }
+		}
+
+	}
+}