fix corner case in unused (#3599)

fixes #3598

lib/compress.js

@@ -455,7 +455,7 @@ merge(Compressor.prototype, {
                 if (def.fixed == null) {
                     if (is_arguments(def)) return false;
                     if (def.global && def.name == "arguments") return false;
-                    def.fixed = make_node(AST_Undefined, def.orig);
+                    def.fixed = make_node(AST_Undefined, def.orig[0]);
                 }
                 return true;
             }
@@ -3870,7 +3870,7 @@ merge(Compressor.prototype, {
                         def.value = null;
                         head.push(def);
                     } else {
-                        var value = def.value && def.value.drop_side_effect_free(compressor);
+                        var value = def.value && !def.value.single_use && def.value.drop_side_effect_free(compressor);
                         if (value) {
                             AST_Node.warn("Side effects in initialization of unused variable {name} [{file}:{line},{col}]", template(def.name));
                             side_effects.push(value);

test/compress/drop-unused.js

@@ -2235,3 +2235,34 @@ function_assign: {
     }
     expect_stdout: "PASS"
 }
+
+issue_3598: {
+    options = {
+        collapse_vars: true,
+        reduce_vars: true,
+        unused: true,
+    }
+    input: {
+        var a = "FAIL";
+        try {
+            (function() {
+                var b = void 0;
+                a = "PASS";
+                c.p = 0;
+                var c = b[!1];
+            })();
+        } catch (e) {}
+        console.log(a);
+    }
+    expect: {
+        var a = "FAIL";
+        try {
+            (function() {
+                a = "PASS";
+                var c = (void (c.p = 0))[!1];
+            })();
+        } catch (e) {}
+        console.log(a);
+    }
+    expect_stdout: "PASS"
+}