fix corner case in unused (#5706)

fixes #5705

lib/compress.js

@@ -7251,15 +7251,13 @@ Compressor.prototype.compress = function(node) {
                 node.name = null;
             }
             if (node instanceof AST_Lambda) {
+                descend_scope();
                 if (drop_funcs && node !== self && node instanceof AST_LambdaDefinition) {
                     var def = node.name.definition();
                     if (!(def.id in in_use_ids)) {
                         log(node.name, "Dropping unused function {name}");
                         def.eliminated++;
-                        if (parent instanceof AST_ExportDefault) {
-                            descend_scope();
-                            return to_func_expr(node, true);
-                        }
+                        if (parent instanceof AST_ExportDefault) return to_func_expr(node, true);
                         return in_list ? List.skip : make_node(AST_EmptyStatement, node);
                     }
                 }
@@ -7357,6 +7355,7 @@ Compressor.prototype.compress = function(node) {
                     }
                     fns_with_marked_args.push(node);
                 }
+                return node;
             }
             if (node instanceof AST_Catch && node.argname instanceof AST_Destructured) {
                 node.argname.transform(trimmer);

test/compress/rests.js

@@ -1624,3 +1624,21 @@ issue_5552_4: {
     expect_stdout: "PASS"
     node_version: ">=6"
 }
+
+issue_5705: {
+    options = {
+        reduce_vars: true,
+        rests: true,
+        unused: true,
+    }
+    input: {
+        (function(...a) {
+            var b = { ...a };
+        })(console.log("PASS"));
+    }
+    expect: {
+        (function() {})(console.log("PASS"));
+    }
+    expect_stdout: "PASS"
+    node_version: ">=8.3.0"
+}

test/compress/side_effects.js

@@ -584,9 +584,7 @@ issue_4668: {
     }
     expect: {
         console.log(function f() {
-            (function g() {
-                0;
-            })();
+            (function g() {})();
         }());
     }
     expect_stdout: "undefined"