fix corner case in unused (#4147)

fixes #4146

.github/workflows/ufuzz.yml

@@ -36,7 +36,7 @@ jobs:
           npm config set update-notifier false
           npm --version
           while !(npm install); do echo "'npm install' failed - retrying..."; done
-          PERIOD=-2000
+          PERIOD=-5000
           if [[ $CAUSE == "schedule" ]]; then
             PERIOD=`node test/ufuzz/actions $BASE_URL $TOKEN`
           fi

lib/compress.js

@@ -6474,17 +6474,16 @@ merge(Compressor.prototype, {
                     side_effects.push(node);
                     args[pos++] = make_sequence(call, side_effects);
                     side_effects = [];
-                } else if (!trim && fn.argnames[i].__unused) {
+                } else if (!trim) {
                     if (side_effects.length) {
-                        node = make_sequence(call, side_effects);
+                        args[pos++] = make_sequence(call, side_effects);
                         side_effects = [];
                     } else {
-                        node = make_node(AST_Number, args[i], {
+                        args[pos++] = make_node(AST_Number, args[i], {
                             value: 0
                         });
+                        continue;
                     }
-                    args[pos++] = node;
-                    continue;
                 }
             } else {
                 side_effects.push(args[i]);

test/compress/drop-unused.js

@@ -2965,3 +2965,30 @@ issue_4144: {
     }
     expect_stdout: "PASS"
 }
+
+issue_4146: {
+    options = {
+        reduce_vars: true,
+        toplevel: true,
+        unused: true,
+    }
+    input: {
+        function f(a, b) {
+            function g() {}
+            var a = g;
+            var c = b;
+            c.p;
+            console.log(typeof a);
+        }
+        f("FAIL", 42);
+    }
+    expect: {
+        (function(a, b) {
+            a = function () {};
+            var c = b;
+            c.p;
+            console.log(typeof a);
+        })(0, 42);
+    }
+    expect_stdout: "function"
+}