fix corner case in merge_vars (#5771)

fixes #5770

lib/compress.js

@@ -6648,36 +6648,50 @@ Compressor.prototype.compress = function(node) {
 
             function walk_cond(condition, consequent, alternative) {
                 var save = segment;
-                var segments = [ save, save ];
+                var segments = scan_branches(condition, consequent, alternative);
+                if (consequent) {
+                    segment = segments[0];
+                    pop();
+                    if (segment !== save) return;
+                }
+                if (alternative) {
+                    segment = segments[1];
+                    pop();
+                    if (segment !== save) return;
+                }
+                segment = save;
+            }
+
+            function scan_branches(condition, consequent, alternative) {
+                var segments = [ segment, segment ];
                 if (condition instanceof AST_Binary) switch (condition.operator) {
                   case "&&":
-                    segments[0] = walk_cond(condition.left, condition.right)[0];
+                    segments[0] = scan_branches(condition.left, condition.right)[0];
                     break;
                   case "||":
                   case "??":
-                    segments[1] = walk_cond(condition.left, null, condition.right)[1];
+                    segments[1] = scan_branches(condition.left, null, condition.right)[1];
                     break;
                   default:
                     condition.walk(tw);
                     break;
                 } else if (condition instanceof AST_Conditional) {
-                    walk_cond(condition.condition, condition.consequent, condition.alternative);
+                    scan_branches(condition.condition, condition.consequent, condition.alternative);
                 } else {
                     condition.walk(tw);
                 }
-                segment = segments[0];
                 if (consequent) {
+                    segment = segments[0];
                     push();
                     consequent.walk(tw);
+                    segments[0] = segment;
                 }
-                segments[0] = segment;
-                segment = segments[1];
                 if (alternative) {
+                    segment = segments[1];
                     push();
                     alternative.walk(tw);
+                    segments[1] = segment;
                 }
-                segments[1] = segment;
-                segment = save;
                 return segments;
             }
         });

test/compress/merge_vars.js

@@ -3889,3 +3889,56 @@ issue_5714: {
         "42",
     ]
 }
+
+issue_5770_1: {
+    options = {
+        dead_code: true,
+        loops: true,
+        merge_vars: true,
+        toplevel: true,
+    }
+    input: {
+        L: do {
+            if (console)
+                for (var a = "FAIL 1"; a; a--)
+                    continue L;
+            var b = "FAIL 2";
+        } while (console.log(b || "PASS"));
+    }
+    expect: {
+        L: do {
+            if (console) {
+                var a = "FAIL 1";
+                if (a)
+                    continue L;
+            }
+            var b = "FAIL 2";
+        } while (console.log(b || "PASS"));
+    }
+    expect_stdout: "PASS"
+}
+
+issue_5770_2: {
+    options = {
+        conditionals: true,
+        dead_code: true,
+        if_return: true,
+        loops: true,
+        merge_vars: true,
+        toplevel: true,
+    }
+    input: {
+        L: do {
+            for (var a = "FAIL 1"; a; a--)
+                continue L;
+            var b = "FAIL 2";
+        } while (console.log(b || "PASS"));
+    }
+    expect: {
+        L: do {
+            var a = "FAIL 1";
+            var b;
+        } while (a || (b = "FAIL 2"), console.log(b || "PASS"));
+    }
+    expect_stdout: "PASS"
+}

test/reduce.js

@@ -363,9 +363,17 @@ module.exports = function reduce_test(testcase, minify_options, reduce_options)
                 node.condition,
                 node.body,
                 node.alternative,
-            ][ (node.start._permute * steps | 0) % 3 ];
+                node,
+            ][ (node.start._permute * steps | 0) % 4 ];
             node.start._permute += step;
-            if (expr) {
+            if (expr === node) {
+                if (node.alternative) {
+                    expr = node.clone();
+                    expr.alternative = null;
+                    CHANGED = true;
+                    return expr;
+                }
+            } else if (expr) {
                 // replace if statement with its condition, then block or else block
                 CHANGED = true;
                 return to_statement(expr);