if-bug fix

src/main/java/chocopy/pa2/TypeChecker.java

@@ -328,8 +328,12 @@ public class TypeChecker extends AbstractNodeAnalyzer<SymbolType> {
 
     @Override
     public SymbolType analyze(IfExpr node) {
+        SymbolType condition_type = node.condition.dispatch(this);
         SymbolType then_type = node.thenExpr.dispatch(this);
         SymbolType else_type = node.elseExpr.dispatch(this);
+        if(!condition_type.equals(BOOL_TYPE)) {
+            err(node, "if_expr should have BOOL_TYPE");
+        }
         if(!then_type.equals(else_type)) {
             err(node, "then_type not equals to else_type. ");
         }