Passes tests

Author: EricCousineau-TRI

include/pybind11/cast.h

@@ -1364,17 +1364,16 @@ template <typename... Ts> class type_caster<std::tuple<Ts...>>
 
 /// Helper class which abstracts away certain actions. Users can provide specializations for
 /// custom holders, but it's only necessary if the type has a non-standard interface.
-template <typename T>
+template <typename T, typename SFINAE = void>
 struct holder_helper {
     static auto get(const T &p) -> decltype(p.get()) { return p.get(); }
 };
 
-// Specialization for `unique_ptr<>` to address #1138.
-template <typename type, typename deleter>
-struct holder_helper<std::unique_ptr<type, deleter>> {
-    using T = std::unique_ptr<type, deleter>;
-    static type* get(const T& p) { return p.get(); }
-    static type* get(T&& p) { return p.release(); }
+// Specialization for move-only holders to address #1138.
+template <typename T>
+struct holder_helper<T, enable_if_t<!is_copy_constructible<T>::value>> {
+  static auto get(const T &p) -> decltype(p.get()) { return p.get(); }
+  static auto get(T &&p) -> decltype(p.get()) { return p.release(); }
 };
 
 /// Type caster for holder types like std::shared_ptr, etc.

tests/test_smart_ptr.cpp

@@ -40,7 +40,8 @@ template <typename T> class huge_unique_ptr {
     uint64_t padding[10];
 public:
     huge_unique_ptr(T *p) : ptr(p) {};
-    T *get() { return ptr.get(); }
+    T *get() const { return ptr.get(); }
+    T* release() { return ptr.release(); }
 };
 PYBIND11_DECLARE_HOLDER_TYPE(T, huge_unique_ptr<T>);
 
@@ -51,7 +52,7 @@ class custom_unique_ptr {
 public:
     custom_unique_ptr(T* p) : impl(p) { }
     T* get() const { return impl.get(); }
-    T* release_ptr() { return impl.release(); }
+    T* release() { return impl.release(); }
 };
 PYBIND11_DECLARE_HOLDER_TYPE(T, custom_unique_ptr<T>);