fix corner case in unused (#4913)

fixes #4912

lib/compress.js

@@ -6030,8 +6030,10 @@ merge(Compressor.prototype, {
                 }
                 if (node instanceof AST_Definitions) {
                     node.definitions.forEach(function(defn) {
-                        var side_effects = defn.value
+                        var value = defn.value;
-                            && (defn.name instanceof AST_Destructured || defn.value.has_side_effects(compressor));
+                        var side_effects = value
+                            && (defn.name instanceof AST_Destructured || value.has_side_effects(compressor));
+                        var shared = side_effects && value.tail_node().operator == "=";
                         defn.name.mark_symbol(function(name) {
                             if (!(name instanceof AST_SymbolDeclaration)) return;
                             var def = name.definition();
@@ -6046,13 +6048,17 @@ merge(Compressor.prototype, {
                                 in_use_ids[def.id] = true;
                                 in_use.push(def);
                             }
-                            if (defn.value) {
+                            if (value) {
-                                if (!side_effects) initializations.add(def.id, defn.value);
+                                if (!side_effects) {
+                                    initializations.add(def.id, value);
+                                } else if (shared) {
+                                    verify_safe_usage(def, true, value_modified[def.id]);
+                                }
                                 assignments.add(def.id, defn);
                             }
                             return true;
                         }, tw);
-                        if (side_effects) defn.value.walk(tw);
+                        if (side_effects) value.walk(tw);
                     });
                     return true;
                 }
@@ -6755,14 +6761,15 @@ merge(Compressor.prototype, {
                     prop.walk(tw);
                 });
                 if (node instanceof AST_Assign) {
-                    var right = get_rhs(node);
+                    var right = get_rhs(node), shared = false;
                     if (init && node.write_only === true && node_def.scope === self && !right.has_side_effects(compressor)) {
                         initializations.add(node_def.id, right);
                     } else {
                         right.walk(tw);
+                        shared = right.tail_node().operator == "=";
                     }
                     if (node.left === sym) {
-                        if (!node.write_only) {
+                        if (!node.write_only || shared) {
                             verify_safe_usage(node_def, true, value_modified[node_def.id]);
                         }
                     } else {

test/compress/drop-unused.js

@@ -2382,7 +2382,7 @@ issue_3664: {
     }
     expect: {
         console.log(function() {
-            var b = (b && console.log("FAIL"), 0, 0);
+            var a, b = (a = (a = [ b && console.log("FAIL") ]).p = 0, 0);
             return "PASS";
         }());
     }
@@ -3388,3 +3388,85 @@ issue_4834: {
     }
     expect_stdout: "PASS"
 }
+
+issue_4912_1: {
+    options = {
+        pure_getters: "strict",
+        reduce_vars: true,
+        toplevel: true,
+        unused: true,
+    }
+    input: {
+        var a = A = function() {};
+        A;
+        a.prototype = {
+            f: function() {
+                console.log("PASS");
+            },
+        };
+        new A().f();
+    }
+    expect: {
+        var a = A = function() {};
+        A;
+        a.prototype = {
+            f: function() {
+                console.log("PASS");
+            },
+        };
+        new A().f();
+    }
+    expect_stdout: "PASS"
+}
+
+issue_4912_2: {
+    options = {
+        pure_getters: "strict",
+        unused: true,
+    }
+    input: {
+        console.log(function() {
+            var g, f = function() {};
+            f.p = {};
+            (g = f.p.q = function() {}).r = "PASS";
+            return f;
+        }().p.q.r);
+    }
+    expect: {
+        console.log(function() {
+            var g, f = function() {};
+            f.p = {};
+            (g = f.p.q = function() {}).r = "PASS";
+            return f;
+        }().p.q.r);
+    }
+    expect_stdout: "PASS"
+}
+
+issue_4912_3: {
+    options = {
+        pure_getters: "strict",
+        reduce_vars: true,
+        side_effects: true,
+        unused: true,
+    }
+    input: {
+        console.log(function(f, g) {
+            f = function() {};
+            f.p = {};
+            g = f.p.q = function() {};
+            g.r = "PASS";
+            return f;
+        }().p.q.r);
+    }
+    expect: {
+        console.log(function(f, g) {
+            f = function() {};
+            f.p = {};
+            g = f.p.q = function() {};
+            g.r = "PASS";
+            return f;
+        }().p.q.r);
+    }
+    expect_stdout: "PASS"
+}