fix corner case in reduce_vars (#5408)

fixes #5407

lib/compress.js

@@ -763,6 +763,22 @@ Compressor.prototype.compress = function(node) {
             };
         }
 
+        function make_fixed_default(compressor, node, save) {
+            var prev_save, prev_seq;
+            return function() {
+                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) {
+                    prev_save = current;
+                    prev_seq = make_sequence(node, [ current, node.value ]);
+                }
+                return prev_seq;
+            };
+        }
+
         function scan_declaration(tw, compressor, lhs, fixed, visit) {
             var scanner = new TreeWalker(function(node) {
                 if (node instanceof AST_DefaultValue) {
@@ -771,14 +787,7 @@ Compressor.prototype.compress = function(node) {
                     node.value.walk(tw);
                     pop(tw);
                     var save = fixed;
-                    if (save) fixed = make_fixed(save, function(value) {
-                        var ev;
-                        if (is_undefined(value, compressor)
-                            || (ev = fuzzy_eval(compressor, value, true)) === undefined) {
-                            return make_sequence(node, [ value, node.value ]);
-                        }
-                        return ev instanceof AST_Node ? node : value;
-                    });
+                    if (save) fixed = make_fixed_default(compressor, node, save);
                     node.name.walk(scanner);
                     fixed = save;
                     return true;

test/compress/default-values.js

@@ -2209,3 +2209,33 @@ issue_5340_3: {
     expect_stdout: "undefined"
     node_version: ">=6"
 }
+
+issue_5407: {
+    options = {
+        evaluate: true,
+        reduce_vars: true,
+    }
+    input: {
+        (function(a) {
+            for (var i = 0; i < 2; i++)
+                (function(b = 4) {
+                    console.log(b);
+                    a = 2;
+                })(a);
+        })();
+    }
+    expect: {
+        (function(a) {
+            for (var i = 0; i < 2; i++)
+                (function(b = 4) {
+                    console.log(b);
+                    a = 2;
+                })(a);
+        })();
+    }
+    expect_stdout: [
+        "4",
+        "2",
+    ]
+    node_version: ">=6"
+}