Change @__setstate__ to __setstate__[non-constructor] based on fe…

…edback by @rainwoodman. Also revise comments.

include/pybind11/pybind11.h

@@ -394,7 +394,7 @@ class cpp_function : public function {
         // it alive.
         auto *rec = unique_rec.get();
 
-        // Note the "@__setstate__" manipulation below.
+        // Note the "__setstate__" manipulation below.
         rec->is_constructor = rec->name != nullptr
                               && (std::strcmp(rec->name, "__init__") == 0
                                   || std::strcmp(rec->name, "__setstate__") == 0);
@@ -410,9 +410,9 @@ class cpp_function : public function {
         /* Create copies of all referenced C-style strings */
         if (rec->name == nullptr) {
             rec->name = guarded_strdup("");
-        } else if (std::strcmp(rec->name, "@__setstate__") == 0) {
+        } else if (std::strcmp(rec->name, "__setstate__[non-constructor]") == 0) {
             // See google/pywrapcc#30094 for background.
-            rec->name = guarded_strdup(rec->name + 1);
+            rec->name = guarded_strdup("__setstate__");
         } else {
             rec->name = guarded_strdup(rec->name);
         }

tests/test_pickling.cpp

@@ -65,10 +65,16 @@ void wrap(py::module m) {
 
 namespace exercise_getinitargs_getstate_setstate {
 
-// Mechanism similar to what was established with Boost.Python:
-//   https://www.boost.org/doc/libs/1_58_0/libs/python/doc/v2/pickle.html
+// Exercise `__setstate__[non-constructor]` (see google/pywrapcc#30094), which
+// was added to support a pickle protocol as established with Boost.Python
+// (in 2002):
+//   https://www.boost.org/doc/libs/1_31_0/libs/python/doc/v2/pickle.html
 // This mechanism was also adopted for PyCLIF:
 //   https://github.com/google/clif/blob/7d388e1de7db5beeb3d7429c18a2776d8188f44f/clif/testing/python/pickle_compatibility.clif
+// The code below exercises the pickle protocol intentionally exactly as used
+// in PyCLIF, to ensure that pybind11 stays fully compatible with existing
+// client code relying on the long-established protocol. (It is impractical to
+// change all such client code.)
 
 class StoreTwoWithState {
 public:
@@ -97,7 +103,7 @@ void wrap(py::module m) {
             },
             py::arg("protocol") = -1)
         .def(
-            "@__setstate__", // See google/pywrapcc#30094 for background.
+            "__setstate__[non-constructor]", // See google/pywrapcc#30094 for background.
             [](StoreTwoWithState *self, const std::string &state) { self->SetState(state); },
             py::arg("state"));
 }