fix nested unused assignments (#2769)

fixes #2768

lib/compress.js

@@ -2817,7 +2817,7 @@ merge(Compressor.prototype, {
                             def.value.walk(tw);
                         }
                         if (def.name.fixed_value() === def.value) {
-                            fixed_ids[node_def.id] = true;
+                            fixed_ids[node_def.id] = def;
                         }
                     }
                 });
@@ -2846,9 +2846,7 @@ merge(Compressor.prototype, {
                         var def = sym.definition();
                         var in_use = def.id in in_use_ids;
                         if (node instanceof AST_Assign) {
-                            if (!in_use
+                            if (!in_use || def.id in fixed_ids && fixed_ids[def.id] !== node) {
-                                || def.id in fixed_ids
-                                    && node.left.fixed_value() !== node.right) {
                                 return maintain_this_binding(parent, node, node.right.transform(tt));
                             }
                         } else if (!in_use) return make_node(AST_Number, node, {
@@ -2902,25 +2900,29 @@ merge(Compressor.prototype, {
                         if (def.value) def.value = def.value.transform(tt);
                         var sym = def.name.definition();
                         if (!drop_vars || sym.id in in_use_ids) {
+                            if (def.value && sym.id in fixed_ids && fixed_ids[sym.id] !== def) {
+                                def.value = def.value.drop_side_effect_free(compressor);
+                            }
                             if (def.name instanceof AST_SymbolVar) {
                                 var var_defs = var_defs_by_id.get(sym.id);
                                 if (var_defs.length > 1 && (!def.value || sym.orig.indexOf(def.name) > sym.eliminated)) {
                                     compressor.warn("Dropping duplicated definition of variable {name} [{file}:{line},{col}]", template(def.name));
                                     if (def.value) {
-                                        side_effects.push(make_node(AST_Assign, def, {
+                                        var assign = make_node(AST_Assign, def, {
                                             operator: "=",
                                             left: make_node(AST_SymbolRef, def.name, def.name),
                                             right: def.value
-                                        }).transform(tt));
+                                        });
+                                        if (fixed_ids[sym.id] === def) {
+                                            fixed_ids[sym.id] = assign;
+                                        }
+                                        side_effects.push(assign.transform(tt));
                                     }
                                     remove(var_defs, def);
                                     sym.eliminated++;
                                     return;
                                 }
                             }
-                            if (def.value && sym.id in fixed_ids && def.name.fixed_value() !== def.value) {
-                                def.value = def.value.drop_side_effect_free(compressor);
-                            }
                             if (def.value) {
                                 if (side_effects.length > 0) {
                                     if (tail.length > 0) {
@@ -3031,7 +3033,7 @@ merge(Compressor.prototype, {
                 if (node instanceof AST_Assign) {
                     node.right.walk(tw);
                     if (node.left.fixed_value() === node.right) {
-                        fixed_ids[node_def.id] = true;
+                        fixed_ids[node_def.id] = node;
                     }
                 }
                 return true;

test/compress/drop-unused.js

@@ -1644,3 +1644,51 @@ cascade_drop_assign: {
     }
     expect_stdout: "PASS"
 }
+
+chained_3: {
+    options = {
+        reduce_vars: true,
+        unused: true,
+    }
+    input: {
+        console.log(function(a, b) {
+            var c = a, c = b;
+            b++;
+            return c;
+        }(1, 2));
+    }
+    expect: {
+        console.log(function(a, b) {
+            var c = b;
+            b++;
+            return c;
+        }(0, 2));
+    }
+    expect_stdout: "2"
+}
+
+issue_2768: {
+    options = {
+        inline: true,
+        reduce_vars: true,
+        sequences: true,
+        side_effects: true,
+        toplevel: true,
+        unused: true,
+    }
+    input: {
+        var a = "FAIL", c = 1;
+        var c = function(b) {
+            var d = b = a;
+            var e = --b + (d && (a = "PASS"));
+        }();
+        console.log(a, typeof c);
+    }
+    expect: {
+        var a = "FAIL";
+        var c = (d = a, 0, void (d && (a = "PASS")));
+        var d;
+        console.log(a, typeof c);
+    }
+    expect_stdout: "PASS undefined"
+}