fix corner case in unused (#4835)

fixes #4834

lib/compress.js

@@ -8427,16 +8427,19 @@ merge(Compressor.prototype, {
         if (!all(args, function(arg) {
             return !(arg instanceof AST_Spread);
         })) return;
+        var argnames = fn.argnames;
         var is_iife = fn === exp && !fn.name;
         if (fn.rest) {
             if (!(is_iife && compressor.option("rests"))) return;
-            var insert = fn.argnames.length;
+            var insert = argnames.length;
             args = args.slice(0, insert);
             while (args.length < insert) args.push(make_node(AST_Undefined, call).optimize(compressor));
             args.push(make_node(AST_Array, call, { elements: call.args.slice(insert) }));
-            call.args = args;
-            fn.argnames = fn.argnames.concat(fn.rest);
+            argnames = argnames.concat(fn.rest);
             fn.rest = null;
+        } else {
+            args = args.slice();
+            argnames = argnames.slice();
         }
         var pos = 0, last = 0;
         var drop_defaults = is_iife && compressor.option("default_values");
@@ -8452,14 +8455,14 @@ merge(Compressor.prototype, {
         } : return_false;
         var side_effects = [];
         for (var i = 0; i < args.length; i++) {
-            var argname = fn.argnames[i];
+            var argname = argnames[i];
             if (drop_defaults && argname instanceof AST_DefaultValue && args[i].is_defined(compressor)) {
-                fn.argnames[i] = argname = argname.name;
+                argnames[i] = argname = argname.name;
             }
             if (!argname || "__unused" in argname) {
                 var node = args[i].drop_side_effect_free(compressor);
                 if (drop_fargs(argname)) {
-                    if (argname) fn.argnames.splice(i, 1);
+                    if (argname) argnames.splice(i, 1);
                     args.splice(i, 1);
                     if (node) side_effects.push(node);
                     i--;
@@ -8481,7 +8484,7 @@ merge(Compressor.prototype, {
                 }
             } else if (drop_fargs(argname, args[i])) {
                 var node = args[i].drop_side_effect_free(compressor);
-                fn.argnames.splice(i, 1);
+                argnames.splice(i, 1);
                 args.splice(i, 1);
                 if (node) side_effects.push(node);
                 i--;
@@ -8493,15 +8496,17 @@ merge(Compressor.prototype, {
             }
             last = pos;
         }
-        for (; i < fn.argnames.length; i++) {
-            if (drop_fargs(fn.argnames[i])) fn.argnames.splice(i--, 1);
+        for (; i < argnames.length; i++) {
+            if (drop_fargs(argnames[i])) argnames.splice(i--, 1);
         }
+        fn.argnames = argnames;
         args.length = last;
+        call.args = args;
         if (!side_effects.length) return;
         var arg = make_sequence(call, side_effects);
-        args.push(args.length < fn.argnames.length ? make_node(AST_UnaryPrefix, call, {
+        args.push(args.length < argnames.length ? make_node(AST_UnaryPrefix, call, {
             operator: "void",
-            expression: arg
+            expression: arg,
         }) : arg);
     }
 

test/compress/drop-unused.js

@@ -3357,3 +3357,34 @@ issue_4806_3: {
     }
     expect_stdout: "PASS"
 }
+
+issue_4834: {
+    options = {
+        inline: true,
+        keep_fargs: false,
+        pure_getters: "strict",
+        reduce_vars: true,
+        side_effects: true,
+        toplevel: true,
+        unused: true,
+    }
+    input: {
+        try {
+            new function(a, b) {
+                b;
+                b.p;
+            }(42);
+        } catch (e) {
+            console.log("PASS");
+        }
+    }
+    expect: {
+        try {
+            void b.p;
+        } catch (e) {
+            console.log("PASS");
+        }
+        var b;
+    }
+    expect_stdout: "PASS"
+}