enhance `unused` (#3662)

3 files changed, 94 insertions, 9 deletions

diff --git a/lib/compress.js b/lib/compress.js
index 38d09068..2bbd9704 100644
--- a/lib/compress.js
+++ b/lib/compress.js
@@ -3879,7 +3879,7 @@ merge(Compressor.prototype, {
                 });
                 return true;
             }
-            return scan_ref_scoped(node, descend);
+            return scan_ref_scoped(node, descend, true);
         });
         self.walk(tw);
         // pass 2: for every used symbol we need to walk its
@@ -3913,6 +3913,11 @@ merge(Compressor.prototype, {
                     if (node instanceof AST_Assign) {
                         if (!in_use || node.left === sym && def.id in fixed_ids && fixed_ids[def.id] !== node) {
                             value = get_rhs(node);
+                            if (node.write_only) {
+                                value = value.drop_side_effect_free(compressor) || make_node(AST_Number, node, {
+                                    value: 0
+                                });
+                            }
                         }
                     } else if (!in_use) {
                         value = make_node(AST_Number, node, {
@@ -4182,7 +4187,7 @@ merge(Compressor.prototype, {
             return rhs.right.has_side_effects(compressor) ? rhs : rhs.right;
         }
 
-        function scan_ref_scoped(node, descend) {
+        function scan_ref_scoped(node, descend, init) {
             var node_def, props = [], sym = assign_as_unused(node, props);
             if (sym && self.variables.get(sym.name) === (node_def = sym.definition())) {
                 props.forEach(function(prop) {
@@ -4191,7 +4196,11 @@ merge(Compressor.prototype, {
                 if (node instanceof AST_Assign) {
                     if (node.write_only === "p" && node.right.may_throw_on_access(compressor)) return;
                     var right = get_rhs(node);
-                    right.walk(tw);
+                    if (init && node.write_only && node_def.scope === self && !right.has_side_effects(compressor)) {
+                        initializations.add(node_def.id, right);
+                    } else {
+                        right.walk(tw);
+                    }
                     if (node.left === sym) {
                         if (!node_def.chained && sym.fixed_value(true) === right) {
                             fixed_ids[node_def.id] = node;
diff --git a/test/compress/drop-unused.js b/test/compress/drop-unused.js
index b9eea0ed..2e131f7d 100644
--- a/test/compress/drop-unused.js
+++ b/test/compress/drop-unused.js
@@ -2254,3 +2254,84 @@ issue_3598: {
     }
     expect_stdout: "PASS"
 }
+
+self_assign: {
+    options = {
+        passes: 2,
+        side_effects: true,
+        unused: true,
+    }
+    input: {
+        function d(a) {
+            a = a;
+        }
+        function e(a, b) {
+            a = b;
+            b = a;
+        }
+        function f(a, b, c) {
+            a = b;
+            b = c;
+            c = a;
+        }
+        function g(a, b, c) {
+            a = a * b + c;
+        }
+    }
+    expect: {
+        function d(a) {}
+        function e(a, b) {}
+        function f(a, b, c) {}
+        function g(a, b, c) {}
+    }
+}
+
+function_argument_reference: {
+    options = {
+        keep_fargs: false,
+        side_effects: true,
+        unused: true,
+    }
+    input: {
+        var a = 1, b = 42;
+        function f(a) {
+            b <<= a;
+        }
+        f();
+        console.log(a, b);
+    }
+    expect: {
+        var a = 1, b = 42;
+        function f(a) {
+            b <<= a;
+        }
+        f();
+        console.log(a, b);
+    }
+    expect_stdout: "1 42"
+}
+
+function_parameter_ie8: {
+    options = {
+        ie8: true,
+        reduce_vars: true,
+        unused: true,
+    }
+    input: {
+        (function() {
+            var a;
+            function f() {
+                console.log("PASS");
+            }
+            f(a = 1 + a);
+        })();
+    }
+    expect: {
+        (function() {
+            (function f() {
+                console.log("PASS");
+            })(0);
+        })();
+    }
+    expect_stdout: "PASS"
+}
diff --git a/test/compress/functions.js b/test/compress/functions.js
index 30569847..3b9def32 100644
--- a/test/compress/functions.js
+++ b/test/compress/functions.js
@@ -266,12 +266,7 @@ issue_2084: {
     }
     expect: {
         var c = 0;
-        !function() {
-            var c;
-            c = 1 + (c = -1),
-            c = 1 + (c = 0),
-            0 !== 23..toString() && (c = 1 + c);
-        }(),
+        23..toString(),
         console.log(c);
     }
     expect_stdout: "0"