fix the overflow at (https://github.com/c-cube/qcheck/issues/408)

CHANGELOG.md

@@ -54,6 +54,7 @@
   - `TestResult.stats` `TestResult.warnings` `TestResult.collect`
 - Clarify documentation for the `int*` and `nat*` generators
 - Disabled duplicated pretty-printed feedback when using `QCheck_alcotest` runner
+- Fixed the overflow bug when `~count > max_int - 200` affecting `Test.{make_cell,make,make_neg}` in both QCheck and QCheck2
 
 
 ## 0.91 (2025-12-21)

src/core/QCheck2.ml

@@ -1618,7 +1618,7 @@ module Test = struct
     let count = global_count count in
     let long_factor = global_long_factor long_factor in
     let positive = not negative in
-    let max_gen = match max_gen with None -> count + 200 | Some x->x in
+    let max_gen = match max_gen with None -> max count (count + 200) | Some x->x in
     {
       law;
       gen;
@@ -1645,7 +1645,7 @@ module Test = struct
     let positive = not negative in
     (* Make a "fake" QCheck2 arbitrary with no shrinking *)
     let fake_gen = Gen.make_primitive ~gen ~shrink:(fun _ -> Seq.empty) in
-    let max_gen = match max_gen with None -> count + 200 | Some x->x in
+    let max_gen = match max_gen with None -> max count (count + 200) | Some x->x in
     {
       law;
       gen = fake_gen;

test/core/QCheck2_unit_tests.ml

@@ -617,6 +617,23 @@ module TestCount = struct
     with
     | _ -> ()
 
+  (* Regression for https://github.com/c-cube/qcheck/issues/408:
+     [make_cell] computed [max_gen = count + 200] without checking for
+     overflow, so [count >= max_int - 199] wrapped to a negative int
+     and [check_cell] silently ran zero iterations. *)
+  let test_count_max_int_no_overflow () =
+    let cell =
+      QCheck2.(Test.make_cell ~name:"never_true" ~count:max_int
+                 Gen.int (fun _ -> false))
+    in
+    let result =
+      QCheck2.Test.check_cell ~rand:(Random.State.make [|0|]) cell
+    in
+    let tested = QCheck2.TestResult.get_count_gen result in
+    Alcotest.(check bool)
+      "check_cell with count=max_int runs at least one iteration"
+      true (tested >= 1)
+
   let tests =
     ("Test.make ~count", Alcotest.[
          test_case "make with custom count" `Quick test_count_10;
@@ -625,6 +642,8 @@ module TestCount = struct
          test_case "make with 0 count" `Quick test_count_0;
          test_case "make with negative count should fail"
            `Quick test_count_negative_fail;
+         test_case "check_cell with count=max_int does not overflow"
+           `Quick test_count_max_int_no_overflow;
        ])
 end