enhance `unused` (#3584)

1 files changed, 47 insertions, 34 deletions

diff --git a/lib/compress.js b/lib/compress.js
index 92e8d3a3..e97427bc 100644
--- a/lib/compress.js
+++ b/lib/compress.js
@@ -3727,10 +3727,20 @@ merge(Compressor.prototype, {
                         });
                     }
                     if (value) {
-                        props.push(value);
-                        return maintain_this_binding(compressor, parent, node, make_sequence(node, props.map(function(prop) {
-                            return prop.transform(tt);
-                        })));
+                        if (parent instanceof AST_Sequence && parent.tail_node() !== node) {
+                            value = value.drop_side_effect_free(compressor);
+                        }
+                        if (value) props.push(value);
+                        switch (props.length) {
+                          case 0:
+                            return MAP.skip;
+                          case 1:
+                            return maintain_this_binding(compressor, parent, node, props[0].transform(tt));
+                          default:
+                            return make_sequence(node, props.map(function(prop) {
+                                return prop.transform(tt);
+                            }));
+                        }
                     }
                 }
             }
@@ -3874,37 +3884,10 @@ merge(Compressor.prototype, {
                     });
                 }
             }
-            // certain combination of unused name + side effect leads to:
-            //    https://github.com/mishoo/UglifyJS2/issues/44
-            //    https://github.com/mishoo/UglifyJS2/issues/1830
-            //    https://github.com/mishoo/UglifyJS2/issues/1838
-            //    https://github.com/mishoo/UglifyJS2/issues/3371
-            // that's an invalid AST.
-            // We fix it at this stage by moving the `var` outside the `for`.
-            if (node instanceof AST_For) {
-                descend(node, this);
-                var block;
-                if (node.init instanceof AST_BlockStatement) {
-                    block = node.init;
-                    node.init = block.body.pop();
-                    block.body.push(node);
-                }
-                if (node.init instanceof AST_Defun) {
-                    if (!block) {
-                        block = make_node(AST_BlockStatement, node, {
-                            body: [ node ]
-                        });
-                    }
-                    block.body.splice(-1, 0, node.init);
-                    node.init = null;
-                } else if (node.init instanceof AST_SimpleStatement) {
-                    node.init = node.init.body;
-                } else if (is_empty(node.init)) {
-                    node.init = null;
-                }
-                return !block ? node : in_list ? MAP.splice(block.body) : block;
-            }
             if (node instanceof AST_LabeledStatement && node.body instanceof AST_For) {
+                // Certain combination of unused name + side effect leads to invalid AST:
+                //    https://github.com/mishoo/UglifyJS2/issues/1830
+                // We fix it at this stage by moving the label inwards, back to the `for`.
                 descend(node, this);
                 if (node.body instanceof AST_BlockStatement) {
                     var block = node.body;
@@ -3934,6 +3917,36 @@ merge(Compressor.prototype, {
                     col  : sym.start.col
                 };
             }
+        }, function(node, in_list) {
+            if (node instanceof AST_For) {
+                // Certain combination of unused name + side effect leads to invalid AST:
+                //    https://github.com/mishoo/UglifyJS2/issues/44
+                //    https://github.com/mishoo/UglifyJS2/issues/1838
+                //    https://github.com/mishoo/UglifyJS2/issues/3371
+                // We fix it at this stage by moving the `var` outside the `for`.
+                var block;
+                if (node.init instanceof AST_BlockStatement) {
+                    block = node.init;
+                    node.init = block.body.pop();
+                    block.body.push(node);
+                }
+                if (node.init instanceof AST_Defun) {
+                    if (!block) {
+                        block = make_node(AST_BlockStatement, node, {
+                            body: [ node ]
+                        });
+                    }
+                    block.body.splice(-1, 0, node.init);
+                    node.init = null;
+                } else if (node.init instanceof AST_SimpleStatement) {
+                    node.init = node.init.body;
+                } else if (is_empty(node.init)) {
+                    node.init = null;
+                }
+                return !block ? node : in_list ? MAP.splice(block.body) : block;
+            } else if (node instanceof AST_Sequence) {
+                if (node.expressions.length == 1) return node.expressions[0];
+            }
         });
         tt.push(compressor.parent());
         self.transform(tt);