Fix braced break/continue

Author: cknitt

compiler/core/lam_compile.ml

@@ -1877,7 +1877,9 @@ let compile output_prefix =
             S.break_ ~label ()
           else S.break_ ()
         in
-        Js_output.make [stmt] ~output_finished:True)
+        (* [break] is accepted inside braced expressions like [{break}], so keep
+           the usual NeedValue invariant even though JS only has a statement form. *)
+        Js_output.make [stmt] ~value:E.undefined ~output_finished:True)
     | Lcontinue -> (
       match lambda_cxt.loop_stack with
       | [] -> assert false
@@ -1892,7 +1894,7 @@ let compile output_prefix =
             S.continue_ ~label ()
           else S.continue_ ()
         in
-        Js_output.make [stmt] ~output_finished:True)
+        Js_output.make [stmt] ~value:E.undefined ~output_finished:True)
     | Lwhile (p, body) -> compile_while p body lambda_cxt
     | Lfor (id, start, finish, direction, body) -> (
       match (direction, finish) with

tests/tests/src/loop_control_test.mjs

@@ -102,6 +102,31 @@ Mocha.describe("Loop_control_test", () => {
       3
     ], Belt_List.toArray(Belt_List.reverse(values)));
   });
+  Mocha.test("braced break expression in value position compiles", () => {
+    let reached = false;
+    while (true) {
+      break;
+      
+    };
+    Test_utils.eq("File \"loop_control_test.res\", line 95, characters 7-14", false, reached);
+  });
+  Mocha.test("braced continue expression in value position compiles", () => {
+    let values = /* [] */0;
+    for (let i = 0; i <= 3; ++i) {
+      if (i === 1) {
+        continue;
+      }
+      values = {
+        hd: i,
+        tl: values
+      };
+    }
+    Test_utils.eq("File \"loop_control_test.res\", line 111, characters 7-14", [
+      0,
+      2,
+      3
+    ], Belt_List.toArray(Belt_List.reverse(values)));
+  });
   Mocha.test("switch inside for targets the loop", () => {
     let values = /* [] */0;
     for (let i = 0; i <= 5; ++i) {
@@ -119,7 +144,7 @@ Mocha.describe("Loop_control_test", () => {
         continue;
       }
     }
-    Test_utils.eq("File \"loop_control_test.res\", line 96, characters 7-14", [
+    Test_utils.eq("File \"loop_control_test.res\", line 125, characters 7-14", [
       0,
       2,
       3
@@ -143,7 +168,7 @@ Mocha.describe("Loop_control_test", () => {
           };
       }
     }
-    Test_utils.eq("File \"loop_control_test.res\", line 117, characters 7-14", [
+    Test_utils.eq("File \"loop_control_test.res\", line 146, characters 7-14", [
       0,
       2,
       3

tests/tests/src/loop_control_test.res

@@ -82,6 +82,35 @@ describe(__MODULE__, () => {
     eq(__LOC__, [0, 2, 3], values.contents->Belt.List.reverse->Belt.List.toArray)
   })
 
+  test("braced break expression in value position compiles", () => {
+    let reached = ref(false)
+    let acceptUnit = (_: unit) => ()
+
+    while true {
+      let x = {break}
+      acceptUnit(x)
+      reached := true
+    }
+
+    eq(__LOC__, false, reached.contents)
+  })
+
+  test("braced continue expression in value position compiles", () => {
+    let values = ref(list{})
+    let acceptUnit = (_: unit) => ()
+
+    for i in 0 to 3 {
+      if i == 1 {
+        let x = {continue}
+        acceptUnit(x)
+      }
+
+      values := list{i, ...values.contents}
+    }
+
+    eq(__LOC__, [0, 2, 3], values.contents->Belt.List.reverse->Belt.List.toArray)
+  })
+
   test("switch inside for targets the loop", () => {
     let values = ref(list{})