From d97df67f8a72042642fdf796523d470649b2e496 Mon Sep 17 00:00:00 2001
From: alexlamsl <alexlamsl@gmail.com>
Date: Tue, 12 Jan 2021 07:18:35 +0800
Subject: [PATCH] fix corner case in `side_effects`

fixes #4540
---
 lib/compress.js                 |  2 +-
 test/compress/default-values.js | 21 +++++++++++++++++++++
 2 files changed, 22 insertions(+), 1 deletion(-)

diff --git a/lib/compress.js b/lib/compress.js
index 5526972f..9d9d3e23 100644
--- a/lib/compress.js
+++ b/lib/compress.js
@@ -8226,7 +8226,7 @@ merge(Compressor.prototype, {
             if (compressor.option("side_effects")
                 && can_drop
                 && all(fn.body, is_empty)
-                && (fn !== exp || fn_name_unused(fn, compressor))
+                && (fn === exp ? fn_name_unused(fn, compressor) : !has_default && !has_destructured)
                 && !(is_arrow(fn) && fn.value)) {
                 return make_sequence(self, convert_args()).optimize(compressor);
             }
diff --git a/test/compress/default-values.js b/test/compress/default-values.js
index 23e31748..985651a3 100644
--- a/test/compress/default-values.js
+++ b/test/compress/default-values.js
@@ -1549,3 +1549,24 @@ issue_4523: {
     expect_stdout: "PASS"
     node_version: ">=6"
 }
+
+issue_4540: {
+    options = {
+        reduce_vars: true,
+        side_effects: true,
+    }
+    input: {
+        console.log(function() {
+            function f([ a = 0 ]) {}
+            f([]);
+        }());
+    }
+    expect: {
+        console.log(function() {
+            function f([ a = 0 ]) {}
+            f([]);
+        }());
+    }
+    expect_stdout: "undefined"
+    node_version: ">=6"
+}