Test: Default Alias in Base Class

This tests that template type aliases for base class templates are
resolved to the same registered types.

Author: ax3l

Diff for: tests/CMakeLists.txt

@@ -153,6 +153,7 @@ set(PYBIND11_TEST_FILES
     test_stl
     test_stl_binders
     test_tagbased_polymorphic
+    test_template_alias_base
     test_thread
     test_union
     test_virtual_functions)

Diff for: tests/test_template_alias_base.cpp

@@ -0,0 +1,24 @@
+#include "pybind11_tests.h"
+
+#include <memory>
+
+template <class T>
+using DefaultAllocator = std::allocator<T>;
+
+template <template <class> class Allocator = DefaultAllocator>
+struct Base {};
+
+template <template <class> class Allocator = DefaultAllocator>
+struct S : public Base<Allocator> {};
+
+// this returns S<DefaultAllocator> even though we register
+// the type S<std::allocator>
+// MSVC and Clang on Windows failed here to detect the registered type
+S<> make_S() { return S<>{}; }
+
+TEST_SUBMODULE(template_alias_base, m) {
+    py::class_<Base<std::allocator>>(m, "B_std");
+    py::class_<S<std::allocator>, Base<std::allocator>>(m, "S_std");
+
+    m.def("make_S", &make_S);
+}

Diff for: tests/test_template_alias_base.py

@@ -0,0 +1,12 @@
+
+from pybind11_tests import template_alias_base as m
+
+
+def test_can_create_variable():
+    v = m.S_std
+    print(v)
+
+
+def test_can_return_variable():
+    v = m.make_S
+    print(v)