fix: report math.sum overflow

crates/jett_comptime/src/interpreter.rs

@@ -8707,7 +8707,14 @@ impl Interpreter {
                         let mut total: i64 = 0;
                         for item in items {
                             match item {
-                                Value::Int64(n) => total += n,
+                                Value::Int64(n) => {
+                                    let Some(next_total) = total.checked_add(*n) else {
+                                        return Some(Err(format!(
+                                            "math.sum: integer overflow: {total} + {n}"
+                                        )));
+                                    };
+                                    total = next_total;
+                                }
                                 _ => {
                                     return Some(Err(
                                         "math.sum: list must contain int64 values".to_string()

crates/jett_driver/tests/fixture_suite.rs

@@ -380,6 +380,14 @@ fn sized_integer_runtime_bounds_reject_variable_arithmetic() {
     }
 }
 
+#[test]
+fn math_sum_reports_overflow() {
+    assert_runtime_fail(
+        "math_sum_overflow.jett",
+        "runtime error: math.sum: integer overflow: 9223372036854775807 + 1",
+    );
+}
+
 #[test]
 fn run_file_capture_stdout_captures_json_runtime_output() {
     assert_run_stdout(

tests/runtime_fail/math_sum_overflow.jett

@@ -0,0 +1,4 @@
+function main() returns nothing:
+    int64 maximum = 9223372036854775807
+    int64 total = math.sum(list(maximum, 1))
+    return nothing