Fix SpEL compilation for non trivial elvis operand

Issue: SPR-17214

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

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2017 the original author or authors.
+ * Copyright 2002-2018 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -77,8 +77,10 @@ public boolean isCompilable() {
 	public void generateCode(MethodVisitor mv, CodeFlow cf) {
 		// exit type descriptor can be null if both components are literal expressions
 		computeExitTypeDescriptor();
+		cf.enterCompilationScope();
 		this.children[0].generateCode(mv, cf);
 		CodeFlow.insertBoxIfNecessary(mv, cf.lastDescriptor().charAt(0));
+		cf.exitCompilationScope();
 		Label elseTarget = new Label();
 		Label endOfIf = new Label();
 		mv.visitInsn(DUP);
@@ -91,10 +93,12 @@ public void generateCode(MethodVisitor mv, CodeFlow cf) {
 		mv.visitJumpInsn(IFEQ, endOfIf);  // if not empty, drop through to elseTarget
 		mv.visitLabel(elseTarget);
 		mv.visitInsn(POP);
+		cf.enterCompilationScope();
 		this.children[1].generateCode(mv, cf);
 		if (!CodeFlow.isPrimitive(this.exitTypeDescriptor)) {
 			CodeFlow.insertBoxIfNecessary(mv, cf.lastDescriptor().charAt(0));
 		}
+		cf.exitCompilationScope();
 		mv.visitLabel(endOfIf);
 		cf.pushDescriptor(this.exitTypeDescriptor);
 	}