Fix bytecode generation for SpEL OpPlus

Without this change the plus operator would fail to
include a CHECKCAST in generated bytecode when it
was compiled in cases where one of the operands
has a runtime type of String but a statically
declared type that was not String (i.e. Object).

Issue: SPR-12426

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

@@ -206,6 +206,9 @@ public class OpPlus extends Operator {
 		else {
 			cf.enterCompilationScope();
 			operand.generateCode(mv,cf);
+			if (cf.lastDescriptor() != "Ljava/lang/String") {
+				mv.visitTypeInsn(CHECKCAST, "java/lang/String");
+			}
 			cf.exitCompilationScope();
 			mv.visitMethodInsn(INVOKEVIRTUAL, "java/lang/StringBuilder", "append", "(Ljava/lang/String;)Ljava/lang/StringBuilder;", false);
 		}

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

@@ -1683,12 +1683,41 @@ public class SpelCompilationCoverageTests extends AbstractExpressionTests {
 		expression = parse("'hello' + 3 + ' spring'");
 		assertEquals("hello3 spring",expression.getValue(new Greeter()));
 		assertCantCompile(expression);
+		
+		expression = parse("object + 'a'");
+		assertEquals("objecta",expression.getValue(new Greeter()));
+		assertCanCompile(expression);
+		assertEquals("objecta",expression.getValue(new Greeter()));
+		
+		expression = parse("'a'+object");
+		assertEquals("aobject",expression.getValue(new Greeter()));
+		assertCanCompile(expression);
+		assertEquals("aobject",expression.getValue(new Greeter()));
+
+		expression = parse("'a'+object+'a'");
+		assertEquals("aobjecta",expression.getValue(new Greeter()));
+		assertCanCompile(expression);
+		assertEquals("aobjecta",expression.getValue(new Greeter()));
+
+		expression = parse("object+'a'+object");
+		assertEquals("objectaobject",expression.getValue(new Greeter()));
+		assertCanCompile(expression);
+		assertEquals("objectaobject",expression.getValue(new Greeter()));
+
+		expression = parse("object+object");
+		assertEquals("objectobject",expression.getValue(new Greeter()));
+		assertCanCompile(expression);
+		assertEquals("objectobject",expression.getValue(new Greeter()));
 	}
 	
 	public static class Greeter {
 		public String getWorld() {
 			return "world"; 
 		}
+		
+		public Object getObject() {
+			return "object";
+		}
 	}
 
 	@Test