Simplify compilation of array indexing in SpEL's Indexer

spring-expression/src/test/java/org/springframework/expression/spel/SpelCompilationCoverageTests.java

@@ -652,6 +652,18 @@ public class SpelCompilationCoverageTests extends AbstractExpressionTests {
 			assertThat(getAst().getExitDescriptor()).isEqualTo("I");
 		}
 
+		@Test
+		void indexIntoSetCannotBeCompiled() {
+			Set<Integer> set = Set.of(42);
+
+			expression = parser.parseExpression("[0]");
+
+			assertThat(expression.getValue(set)).isEqualTo(42);
+			assertCannotCompile(expression);
+			assertThat(expression.getValue(set)).isEqualTo(42);
+			assertThat(getAst().getExitDescriptor()).isNull();
+		}
+
 		@Test
 		void indexIntoStringCannotBeCompiled() {
 			String text = "enigma";