Support casts of const-qualified unique_ptrs (#988)

This uses the same technique that the `shared_ptr` type caster
does. The updated test previously failed because of an invalid
conversion from `const void*` to `void*`

Author: tjstum

include/nanobind/stl/unique_ptr.h

@@ -48,6 +48,7 @@ struct type_caster<std::unique_ptr<T, Deleter>> {
     static constexpr bool IsClass = true;
     using Value = std::unique_ptr<T, Deleter>;
     using Caster = make_caster<T>;
+    using Td = std::decay_t<T>;
 
     static constexpr bool IsDefaultDeleter =
         std::is_same_v<Deleter, std::default_delete<T>>;
@@ -116,18 +117,18 @@ struct type_caster<std::unique_ptr<T, Deleter>> {
         if constexpr (IsNanobindDeleter)
             cpp_delete = value.get_deleter().owned_by_cpp();
 
-        T *ptr = value.get();
-        const std::type_info *type = &typeid(T);
+        Td *ptr = (Td *) value.get();
+        const std::type_info *type = &typeid(Td);
         if (!ptr)
             return none().release();
 
         constexpr bool has_type_hook =
-            !std::is_base_of_v<std::false_type, type_hook<T>>;
+            !std::is_base_of_v<std::false_type, type_hook<Td>>;
         if constexpr (has_type_hook)
-            type = type_hook<T>::get(ptr);
+            type = type_hook<Td>::get(ptr);
 
         handle result;
-        if constexpr (!std::is_polymorphic_v<T>) {
+        if constexpr (!std::is_polymorphic_v<Td>) {
             result = nb_type_put_unique(type, ptr, cleanup, cpp_delete);
         } else {
             const std::type_info *type_p =
@@ -161,7 +162,7 @@ struct type_caster<std::unique_ptr<T, Deleter>> {
             !nb_type_relinquish_ownership(src.ptr(), IsDefaultDeleter))
             throw next_overload();
 
-        T *p = caster.operator T *();
+        Td *p = caster.operator Td *();
 
         Value value;
         if constexpr (IsNanobindDeleter)

tests/test_holders.cpp

@@ -154,7 +154,7 @@ NB_MODULE(test_holders_ext, m) {
     // ------- unique_ptr -------
 
     m.def("unique_from_cpp",
-          [](int val) { return std::make_unique<Example>(val); },
+          [](int val) { return std::make_unique<const Example>(val); },
           nb::arg() = 1);
     m.def("unique_from_cpp_2", []() {
         return std::unique_ptr<Example, nb::deleter<Example>>(new Example(2));