chore: fix IsBoundVisitor to error on AlwaysTrue & AlwaysFalse (#503)

Author: zhjwpku

src/iceberg/expression/binder.cc

@@ -85,9 +85,13 @@ Result<bool> IsBoundVisitor::IsBound(const std::shared_ptr<Expression>& expr) {
   return Visit<bool, IsBoundVisitor>(expr, visitor);
 }
 
-Result<bool> IsBoundVisitor::AlwaysTrue() { return true; }
+Result<bool> IsBoundVisitor::AlwaysTrue() {
+  return InvalidExpression("IsBoundVisitor does not support AlwaysTrue expression");
+}
 
-Result<bool> IsBoundVisitor::AlwaysFalse() { return true; }
+Result<bool> IsBoundVisitor::AlwaysFalse() {
+  return InvalidExpression("IsBoundVisitor does not support AlwaysFalse expression");
+}
 
 Result<bool> IsBoundVisitor::Not(bool child_result) { return child_result; }
 

src/iceberg/test/expression_visitor_test.cc

@@ -296,14 +296,14 @@ TEST_F(BinderTest, ErrorNestedUnboundField) {
 class IsBoundVisitorTest : public ExpressionVisitorTest {};
 
 TEST_F(IsBoundVisitorTest, Constants) {
-  // True and False are always considered bound
+  // True and False should error out
   auto true_expr = Expressions::AlwaysTrue();
-  ICEBERG_UNWRAP_OR_FAIL(auto is_bound_true, IsBoundVisitor::IsBound(true_expr));
-  EXPECT_TRUE(is_bound_true);
+  auto result_true = IsBoundVisitor::IsBound(true_expr);
+  EXPECT_THAT(result_true, IsError(ErrorKind::kInvalidExpression));
 
   auto false_expr = Expressions::AlwaysFalse();
-  ICEBERG_UNWRAP_OR_FAIL(auto is_bound_false, IsBoundVisitor::IsBound(false_expr));
-  EXPECT_TRUE(is_bound_false);
+  auto result_false = IsBoundVisitor::IsBound(false_expr);
+  EXPECT_THAT(result_false, IsError(ErrorKind::kInvalidExpression));
 }
 
 TEST_F(IsBoundVisitorTest, UnboundPredicate) {