Fix apparent nondeterminism in pattern matching compilation.

CHANGELOG.md

@@ -39,6 +39,7 @@
 - Prop punning when types don't match results in I/O error: _none_: No such file or directory. https://github.com/rescript-lang/rescript/pull/7533
 - Fix partial application with user-defined function types. https://github.com/rescript-lang/rescript/pull/7548
 - Fix doc comment before variant throwing syntax error. https://github.com/rescript-lang/rescript/pull/7535
+- Fix apparent non-determinism in generated code for pattern matching. https://github.com/rescript-lang/rescript/pull/7557
 
 #### :nail_care: Polish
 

compiler/ml/matching.ml

@@ -1808,7 +1808,10 @@ let reintroduce_fail sw =
     let i_max = ref (-1) and max = ref (-1) in
     Hashtbl.iter
       (fun i c ->
-        if c > !max then (
+        if
+          c > !max || (c = !max && i > !i_max)
+          (* tie-break for determinism: choose the smallest index*)
+        then (
           i_max := i;
           max := c))
       t;

tests/tests/src/NondetPatterMatch.mjs

@@ -0,0 +1,36 @@
+// Generated by ReScript, PLEASE EDIT WITH CARE
+
+
+function f1(action) {
+  if (typeof action === "object") {
+    switch (action.TAG) {
+      case "WithPayload5" :
+      case "WithPayload6" :
+      case "WithPayload7" :
+      case "WithPayload8" :
+        console.log("hello");
+        break;
+    }
+  }
+  return 42;
+}
+
+function f2(action) {
+  if (typeof action === "object") {
+    switch (action.TAG) {
+      case "WithPayload5" :
+      case "WithPayload6" :
+      case "WithPayload7" :
+      case "WithPayload8" :
+        console.log("hello");
+        break;
+    }
+  }
+  return 42;
+}
+
+export {
+  f1,
+  f2,
+}
+/* No side effect */

tests/tests/src/NondetPatterMatch.res

@@ -0,0 +1,33 @@
+type action =
+  | WithoutPayload1
+  | WithoutPayload2
+  | WithPayload2({y: int})
+  | WithPayload3({y: int})
+  | WithPayload5({y: int})
+  | WithPayload6({x: int})
+  | WithPayload7({y: int})
+  | WithPayload8({x: int})
+
+let f1 = (action: action) => {
+  switch action {
+  | WithPayload5(_)
+  | WithPayload6(_)
+  | WithPayload7(_)
+  | WithPayload8(_) =>
+    Console.log("hello")
+  | _ => ()
+  }
+  42
+}
+
+let f2 = (action: action) => {
+  switch action {
+  | WithPayload5(_)
+  | WithPayload6(_)
+  | WithPayload7(_)
+  | WithPayload8(_) =>
+    Console.log("hello")
+  | _ => ()
+  }
+  42
+}

tests/tests/src/adt_optimize_test.mjs

@@ -134,19 +134,22 @@ function f8(x) {
 
 function f9(x) {
   if (typeof x !== "object") {
-    if (x === "T63") {
-      return 3;
-    } else {
-      return 1;
+    switch (x) {
+      case "T60" :
+      case "T61" :
+      case "T62" :
+        return 1;
+      default:
+        return 3;
+    }
+  } else {
+    switch (x.TAG) {
+      case "T64" :
+      case "T65" :
+        return 2;
+      default:
+        return 3;
     }
-  }
-  switch (x.TAG) {
-    case "T64" :
-    case "T65" :
-      return 2;
-    case "T66" :
-    case "T68" :
-      return 3;
   }
 }