Ensure cast correctly included for OpPlus compilation

When the plus operator is used between strings in a SpEL
expression and that expression is compiled, it is
important to include a cast if computation of any of
the operands isn't obviously leaving strings on the
stack. Likewise if the stack contents are known to
be strings, a cast should not be included.

Issue: SPR-12426

spring-expression/src/main/java/org/springframework/expression/spel/ast/OpPlus.java

@@ -198,7 +198,7 @@ public class OpPlus extends Operator {
 		else {
 			cf.enterCompilationScope();
 			operand.generateCode(mv,cf);
-			if (cf.lastDescriptor() != "Ljava/lang/String") {
+			if (!"Ljava/lang/String".equals(cf.lastDescriptor())) {
 				mv.visitTypeInsn(CHECKCAST, "java/lang/String");
 			}
 			cf.exitCompilationScope();

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

@@ -4194,6 +4194,29 @@ public class SpelCompilationCoverageTests extends AbstractExpressionTests {
 		assertEquals("value1",stringify(expression.getValue(mapArray)));
 		assertEquals("Ljava/lang/Object",getAst().getExitDescriptor());
 	}
+	
+	@Test
+	public void plusNeedingCheckcast_SPR12426() {
+		expression = parser.parseExpression("object + ' world'");
+		Object v = expression.getValue(new FooObject());
+		assertEquals("hello world",v);
+		assertCanCompile(expression);
+		assertEquals("hello world",v);		
+		
+		expression = parser.parseExpression("object + ' world'");
+		v = expression.getValue(new FooString());
+		assertEquals("hello world",v);
+		assertCanCompile(expression);
+		assertEquals("hello world",v);		
+	}
+	
+	public static class FooObject {
+		public Object getObject() { return "hello"; }
+	}
+
+	public static class FooString {
+		public String getObject() { return "hello"; }
+	}
 
 	@Test
 	public void mixingItUp_propertyAccessIndexerOpLtTernaryRootNull() throws Exception {