note: cast from 'void *' is not allowed in a constant expression because the pointed object type 'std::__tree_node<std::__value_type<int, double>, min_pointer<void>>' is not similar to the target type 'std::__tree_node_base<min_pointer<void>>

Author: vinay-deshmukh

libcxx/include/__tree

@@ -1800,12 +1800,65 @@ pair<typename __tree<_Tp, _Compare, _Allocator>::iterator, bool>
 _LIBCPP_CONSTEXPR_SINCE_CXX26 __tree<_Tp, _Compare, _Allocator>::__emplace_hint_unique_key_args(
     const_iterator __p, _Key const& __k, _Args&&... __args) {
   __parent_pointer __parent;
+  // std::__tree_end_node<std::__tree_node_base<void *> *>
   __node_base_pointer __dummy;
+  // min_pointer<std::__tree_node_base<min_pointer<void>>>
+
   __node_base_pointer& __child = __find_equal(__p, __parent, __dummy, __k);
   __node_pointer __r           = static_cast<__node_pointer>(__child);
   bool __inserted              = false;
   if (__child == nullptr) {
     __node_holder __h = __construct_node(std::forward<_Args>(__args)...);
+    // std::unique_ptr<std::__tree_node<std::__value_type<int, double>, min_pointer<void>>, std::__tree_node_destructor<min_allocator<std::__tree_node<std::__value_type<int, double>, min_pointer<void>>>>>
+
+    // *h
+    // std::__tree_node<std::__value_type<int, double>, min_pointer<void>>
+    // (*__h).fooooo;
+    // std::__tree_node<std::__value_type<int, double>, min_pointer<void>>
+
+    // __h.get().foooooo;
+    // min_pointer<std::__tree_node<std::__value_type<int, double>, min_pointer<void>>>
+
+    // __node_base_pointer  = min_pointer<std::__tree_node_base<                                min_pointer<void>>>
+    // __h.get()            = min_pointer<std::__tree_node     <std::__value_type<int, double>, min_pointer<void>>>
+
+
+    // static_assert();
+
+    // __tree_node::__node_value_type = std::__value_type<int, double>
+
+    /*
+    without 233
+    no matching conversion for static_cast
+    from 'pointer' (aka
+      'min_pointer<std::__tree_node<std::__value_type<int, double>, min_pointer<void, std::integral_constant<unsigned long, 0>>>>'
+    ) 
+    to '__node_base_pointer' (aka 
+      'min_pointer<std::__tree_node_base<min_pointer<void, std::integral_constant<unsigned long, 0>>>, std::integral_constant<unsigned long, 0>>'
+    )
+    */
+
+
+
+
+    [[maybe_unused]] __node_base_pointer foo = static_cast<__node_base_pointer>(__h.get());
+    // [[maybe_unused]] __node_base_pointer foo2 = __node_base_pointer(__h.get());
+
+
+
+
+    /*
+    note:
+    cast from 'void *' is not allowed in a constant expression
+    because the pointed object type
+      'std::__tree_node<std::__value_type<int, double>, min_pointer<void>>'
+    is not similar to the target type
+      'std::__tree_node_base<min_pointer<void>>'
+    */
+
+// i believe this cast fails because the lifetime of __h.get() has not begun
+
+
     __insert_node_at(__parent, __child, static_cast<__node_base_pointer>(__h.get()));
     __r        = __h.release();
     __inserted = true;

libcxx/test/std/containers/associative/map/map.access/at.pass.cpp

@@ -16,6 +16,7 @@
 #include <cassert>
 #include <map>
 #include <stdexcept>
+#include <type_traits>
 
 #include "min_allocator.h"
 #include "test_macros.h"
@@ -100,6 +101,25 @@ TEST_CONSTEXPR_CXX26 bool test() {
         V(7, 7.5),
         V(8, 8.5),
     };
+
+    // std::__tree_node<std::__value_type<int,double>, min_pointer<void>> d;
+    // std::__tree_node_base<min_pointer<void>> b  = d;
+    using Base = std::__tree_node_base<min_pointer<void>>;
+
+    using Derived = std::__tree_node<std::__value_type<int,double>, min_pointer<void>> ;
+    static_assert(std::is_base_of_v<Base, Derived>);
+
+
+    using BaseP = min_pointer<Base>;
+    using DerivedP = min_pointer<Derived>;
+    // static_assert(std::is_base_of_v<BaseP, DerivedP>);
+    DerivedP dp(nullptr);
+
+    BaseP bp =static_cast<BaseP>(dp); 
+    (void)bp;
+
+
+
     std::map<int, double, std::less<int>, min_allocator<V>> m(ar, ar + sizeof(ar) / sizeof(ar[0]));
     assert(m.size() == 7);
     assert(m.at(1) == 1.5);