fix corner case in unused (#4145)

fixes #4144

lib/compress.js

@@ -6474,7 +6474,7 @@ merge(Compressor.prototype, {
                     side_effects.push(node);
                     args[pos++] = make_sequence(call, side_effects);
                     side_effects = [];
-                } else if (!trim) {
+                } else if (!trim && fn.argnames[i].__unused) {
                     if (side_effects.length) {
                         node = make_sequence(call, side_effects);
                         side_effects = [];

test/compress/drop-unused.js

@@ -2945,3 +2945,23 @@ issue_4133: {
     }
     expect_stdout: "0"
 }
+
+issue_4144: {
+    options = {
+        keep_fargs: "strict",
+        reduce_vars: true,
+        unused: true,
+    }
+    input: {
+        (function(a, b) {
+            var b = console, c = ++b;
+        })(console.log("PASS"), 0);
+    }
+    expect: {
+        (function(b) {
+            b = console,
+            ++b;
+        })(console.log("PASS"));
+    }
+    expect_stdout: "PASS"
+}