fix corner case in merge_vars (#4407)

fixes #4406

lib/compress.js

@@ -4801,7 +4801,7 @@ merge(Compressor.prototype, {
             if (node instanceof AST_Call) {
                 var exp = node.expression;
                 var tail = exp.tail_node();
-                if (!(tail instanceof AST_Function)) return;
+                if (!is_function(tail)) return;
                 if (exp !== tail) exp.expressions.slice(0, -1).forEach(function(node) {
                     node.walk(tw);
                 });

test/compress/async.js

@@ -567,3 +567,33 @@ issue_4377: {
     expect_stdout: "function"
     node_version: ">=8"
 }
+
+issue_4406: {
+    options = {
+        merge_vars: true,
+    }
+    input: {
+        A = "PASS";
+        B = "FAIL";
+        (function() {
+            var a, b;
+            a = A;
+            (async function({
+                [console.log(a)]: {},
+            }) {})((b = B) && { undefined: b });
+        })();
+    }
+    expect: {
+        A = "PASS";
+        B = "FAIL";
+        (function() {
+            var a, b;
+            a = A;
+            (async function({
+                [console.log(a)]: {},
+            }) {})((b = B) && { undefined: b });
+        })();
+    }
+    expect_stdout: "PASS"
+    node_version: ">=8"
+}