fix corner case in merge_vars (#5773)

fixes #5772

Author: alexlamsl

lib/compress.js

@@ -6648,49 +6648,58 @@ Compressor.prototype.compress = function(node) {
 
             function walk_cond(condition, consequent, alternative) {
                 var save = segment;
-                var segments = scan_branches(condition, consequent, alternative);
+                var segments = scan_branches(1, condition, consequent, alternative);
                 if (consequent) {
-                    segment = segments[0];
-                    pop();
+                    segment = segments.consequent.segment;
+                    for (var i = segments.consequent.level; --i >= 0;) pop();
                     if (segment !== save) return;
                 }
                 if (alternative) {
-                    segment = segments[1];
-                    pop();
+                    segment = segments.alternative.segment;
+                    for (var i = segments.alternative.level; --i >= 0;) pop();
                     if (segment !== save) return;
                 }
                 segment = save;
             }
 
-            function scan_branches(condition, consequent, alternative) {
-                var segments = [ segment, segment ];
+            function scan_branches(level, condition, consequent, alternative) {
+                var segments = {
+                    consequent: {
+                        segment: segment,
+                        level: level,
+                    },
+                    alternative: {
+                        segment: segment,
+                        level: level,
+                    },
+                }
                 if (condition instanceof AST_Binary) switch (condition.operator) {
                   case "&&":
-                    segments[0] = scan_branches(condition.left, condition.right)[0];
+                    segments.consequent = scan_branches(level + 1, condition.left, condition.right).consequent;
                     break;
                   case "||":
                   case "??":
-                    segments[1] = scan_branches(condition.left, null, condition.right)[1];
+                    segments.alternative = scan_branches(level + 1, condition.left, null, condition.right).alternative;
                     break;
                   default:
                     condition.walk(tw);
                     break;
                 } else if (condition instanceof AST_Conditional) {
-                    scan_branches(condition.condition, condition.consequent, condition.alternative);
+                    scan_branches(level + 1, condition.condition, condition.consequent, condition.alternative);
                 } else {
                     condition.walk(tw);
                 }
                 if (consequent) {
-                    segment = segments[0];
+                    segment = segments.consequent.segment;
                     push();
                     consequent.walk(tw);
-                    segments[0] = segment;
+                    segments.consequent.segment = segment;
                 }
                 if (alternative) {
-                    segment = segments[1];
+                    segment = segments.alternative.segment;
                     push();
                     alternative.walk(tw);
-                    segments[1] = segment;
+                    segments.alternative.segment = segment;
                 }
                 return segments;
             }

test/compress/merge_vars.js

@@ -3942,3 +3942,61 @@ issue_5770_2: {
     }
     expect_stdout: "PASS"
 }
+
+issue_5772_1: {
+    options = {
+        dead_code: true,
+        merge_vars: true,
+        loops: true,
+    }
+    input: {
+        (function(a) {
+            while (--a)
+                return;
+            var b = console.log("foo") && (c = 42) ? 0 : console.log(c);
+            var c = b;
+        })();
+    }
+    expect: {
+        (function(a) {
+            if (--a)
+                return;
+            var a = console.log("foo") && (c = 42) ? 0 : console.log(c);
+            var c = a;
+        })();
+    }
+    expect_stdout: [
+        "foo",
+        "undefined",
+    ]
+}
+
+issue_5772_2: {
+    options = {
+        dead_code: true,
+        merge_vars: true,
+        loops: true,
+    }
+    input: {
+        (function(a) {
+            while (--a)
+                return;
+            var b;
+            var c = console.log("foo") && (b = 1) ? 2 : 3;
+            console.log(b, c);
+        })();
+    }
+    expect: {
+        (function(a) {
+            if (--a)
+                return;
+            var b;
+            var a = console.log("foo") && (b = 1) ? 2 : 3;
+            console.log(b, a);
+        })();
+    }
+    expect_stdout: [
+        "foo",
+        "undefined 3",
+    ]
+}