fix corner case in reduce_vars (#5464)

fixes #5463

lib/compress.js

@@ -766,14 +766,14 @@ Compressor.prototype.compress = function(node) {
         function make_fixed_default(compressor, node, save) {
             var prev_save, prev_seq;
             return function() {
+                if (prev_seq === node) return node;
                 var current = save();
-                var ev;
-                if (!is_undefined(current, compressor) && (ev = fuzzy_eval(compressor, current, true)) !== undefined) {
-                    return ev instanceof AST_Node ? node : current;
-                }
-                if (prev_save !== current) {
+                var ev = fuzzy_eval(compressor, current, true);
+                if (ev instanceof AST_Node) {
+                    prev_seq = node;
+                } else if (prev_save !== current) {
                     prev_save = current;
-                    prev_seq = make_sequence(node, [ current, node.value ]);
+                    prev_seq = ev === undefined ? make_sequence(node, [ current, node.value ]) : current;
                 }
                 return prev_seq;
             };
@@ -9152,6 +9152,7 @@ Compressor.prototype.compress = function(node) {
 
     function fuzzy_eval(compressor, node, nullish) {
         if (node.truthy) return true;
+        if (is_undefined(node)) return undefined;
         if (node.falsy && !nullish) return false;
         if (node.is_truthy()) return true;
         return node.evaluate(compressor, true);

test/compress/default-values.js

@@ -2395,3 +2395,29 @@ issue_5448_4: {
     expect_stdout: "PASS"
     node_version: ">=6"
 }
+
+issue_5463: {
+    options = {
+        collapse_vars: true,
+        conditionals: true,
+        evaluate: true,
+        reduce_vars: true,
+        toplevel: true,
+        unsafe: true,
+    }
+    input: {
+        if (console.log("PASS"))
+            var a = void 0,
+                b = void 0,
+                b = ([ a = FAIL ] = b && b);
+    }
+    expect: {
+        var a, b, b;
+        console.log("PASS") && (
+            b = a = void 0,
+            b = [a = FAIL] = a && a
+        );
+    }
+    expect_stdout: "PASS"
+    node_version: ">=6"
+}