From d2c50ace997da8b1f7a3b0bd75bd1b376907928a Mon Sep 17 00:00:00 2001
From: "Alex Lam S.L" <alexlamsl@gmail.com>
Date: Fri, 18 Dec 2020 04:20:43 +0000
Subject: [PATCH] fix corner case in `merge_vars` (#4402)

fixes #4401
---
 lib/compress.js         |  6 +++++-
 test/compress/arrows.js | 25 +++++++++++++++++++++++++
 test/ufuzz/index.js     |  2 +-
 3 files changed, 31 insertions(+), 2 deletions(-)

diff --git a/lib/compress.js b/lib/compress.js
index 33d8cf55..ab5ad06d 100644
--- a/lib/compress.js
+++ b/lib/compress.js
@@ -4915,7 +4915,11 @@ merge(Compressor.prototype, {
                         argname.mark_symbol(marker, scanner);
                     });
                 }
-                walk_body(node, tw);
+                if (node instanceof AST_Arrow && node.value) {
+                    node.value.walk(tw);
+                } else {
+                    walk_body(node, tw);
+                }
                 pop();
                 return true;
             }
diff --git a/test/compress/arrows.js b/test/compress/arrows.js
index dd71fe9b..c91cd275 100644
--- a/test/compress/arrows.js
+++ b/test/compress/arrows.js
@@ -465,3 +465,28 @@ issue_4390: {
     ]
     node_version: ">=4"
 }
+
+issue_4401: {
+    options = {
+        merge_vars: true,
+    }
+    input: {
+        (function() {
+            var a = (b => b(a))(console.log || a);
+            var c = console.log;
+            c && c(typeof b);
+        })();
+    }
+    expect: {
+        (function() {
+            var a = (b => b(a))(console.log || a);
+            var c = console.log;
+            c && c(typeof b);
+        })();
+    }
+    expect_stdout: [
+        "undefined",
+        "undefined",
+    ]
+    node_version: ">=4"
+}
diff --git a/test/ufuzz/index.js b/test/ufuzz/index.js
index c4ad4a78..eb38a759 100644
--- a/test/ufuzz/index.js
+++ b/test/ufuzz/index.js
@@ -1011,7 +1011,7 @@ function _createExpression(recurmax, noComma, stmtDepth, canThrow) {
                         suffix = "})";
                     } else {
                         s.push("((" + params + ") => ");
-                        switch (rng(4)) {
+                        switch (rng(10)) {
                           case 0:
                             s.push('(typeof arguments != "undefined" && arguments && arguments[' + rng(3) + "])");
                             break;
-- 
2.34.1