fix: enable bind_map with using declarations.

include/pybind11/stl_bind.h

@@ -525,7 +525,7 @@ class_<Vector, holder_type> bind_vector(handle scope, std::string const &name, A
         [](const Vector &v) -> bool { return !v.empty(); },
         "Check whether the list is nonempty");
 
-    cl.def("__len__", &Vector::size);
+    cl.def("__len__", [](const Vector &vec) { return vec.size(); });
 
 #if 0
     // C++ style functions deprecated, leaving it here as an example
@@ -843,7 +843,8 @@ class_<Map, holder_type> bind_map(handle scope, const std::string &name, Args &&
         m.erase(it);
     });
 
-    cl.def("__len__", &Map::size);
+    // Always use a lambda in case of `using` declaration
+    cl.def("__len__", [](const Map &m) { return m.size(); });
 
     return cl;
 }

tests/test_stl_binders.cpp

@@ -15,6 +15,7 @@
 #include <deque>
 #include <map>
 #include <unordered_map>
+#include <vector>
 
 class El {
 public:
@@ -83,6 +84,69 @@ struct RecursiveMap : std::map<int, RecursiveMap> {
     using Parent::Parent;
 };
 
+class UserVectorLike : private std::vector<int> {
+public:
+    using Base = std::vector<int>;
+    using typename Base::const_iterator;
+    using typename Base::difference_type;
+    using typename Base::iterator;
+    using typename Base::size_type;
+    using typename Base::value_type;
+
+    using Base::at;
+    using Base::back;
+    using Base::Base;
+    using Base::begin;
+    using Base::cbegin;
+    using Base::cend;
+    using Base::clear;
+    using Base::empty;
+    using Base::end;
+    using Base::erase;
+    using Base::front;
+    using Base::insert;
+    using Base::pop_back;
+    using Base::push_back;
+    using Base::reserve;
+    using Base::shrink_to_fit;
+    using Base::swap;
+    using Base::operator[];
+    using Base::capacity;
+    using Base::size;
+};
+
+bool operator==(UserVectorLike const &, UserVectorLike const &) { return true; }
+bool operator!=(UserVectorLike const &, UserVectorLike const &) { return false; }
+
+class UserMapLike : private std::map<int, int> {
+public:
+    using Base = std::map<int, int>;
+    using typename Base::const_iterator;
+    using typename Base::iterator;
+    using typename Base::key_type;
+    using typename Base::mapped_type;
+    using typename Base::size_type;
+    using typename Base::value_type;
+
+    using Base::at;
+    using Base::Base;
+    using Base::begin;
+    using Base::cbegin;
+    using Base::cend;
+    using Base::clear;
+    using Base::emplace;
+    using Base::emplace_hint;
+    using Base::empty;
+    using Base::end;
+    using Base::erase;
+    using Base::find;
+    using Base::insert;
+    using Base::max_size;
+    using Base::swap;
+    using Base::operator[];
+    using Base::size;
+};
+
 /*
  * Pybind11 does not catch more complicated recursion schemes, such as mutual
  * recursion.
@@ -173,6 +237,11 @@ TEST_SUBMODULE(stl_binders, m) {
     py::bind_map<MutuallyRecursiveContainerPairMV>(m, "MutuallyRecursiveContainerPairMV");
     py::bind_vector<MutuallyRecursiveContainerPairVM>(m, "MutuallyRecursiveContainerPairVM");
 
+    // Bind with private inheritance + `using` directives.
+    // Feel free to add new `using` directives there.
+    py::bind_vector<UserVectorLike>(m, "UserVectorLike");
+    py::bind_map<UserMapLike>(m, "UserMapLike");
+
     // The rest depends on numpy:
     try {
         py::module_::import("numpy");