fix corner case in side_effects (#4541)

fixes #4540

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);
             }

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"
+}