first attempt

Author: Roy

include/boost/json/detail/value_to.hpp

@@ -12,6 +12,28 @@
 #ifndef BOOST_JSON_DETAIL_VALUE_TO_HPP
 #define BOOST_JSON_DETAIL_VALUE_TO_HPP
 
+
+#ifndef BOOST_JSON_DETAIL_VALUE_TO_HPP
+#define BOOST_JSON_DETAIL_VALUE_TO_HPP
+#endif
+
+#ifndef BOOST_JSON_INTRUSIVE_INDEX_INC
+#define BOOST_JSON_INTRUSIVE_INDEX_INC ((void)0);
+#endif
+
+#ifndef BOOST_JSON_INTRUSIVE_PATH_PUSH
+#define BOOST_JSON_INTRUSIVE_PATH_PUSH(x) ((void)0);
+#endif
+
+#ifndef BOOST_JSON_INTRUSIVE_PATH_POP
+#define BOOST_JSON_INTRUSIVE_PATH_POP ((void)0);
+#endif
+
+#ifndef BOOST_JSON_INTRUSIVE_MESSAGE
+#define BOOST_JSON_INTRUSIVE_MESSAGE(x) ((void)0);
+#endif
+
+
 #include <boost/json/value.hpp>
 #include <boost/json/conversion.hpp>
 #include <boost/json/result_for.hpp>
@@ -270,13 +292,21 @@ value_to_impl(
     }
 
     auto ins = detail::inserter(result, inserter_implementation<T>());
+
+    BOOST_JSON_INTRUSIVE_PATH_PUSH(-1)
+
     for( value const& val: *arr )
     {
+        BOOST_JSON_INTRUSIVE_INDEX_INC
+
         auto elem_res = try_value_to<value_type<T>>( val, ctx );
         if( elem_res.has_error() )
             return {boost::system::in_place_error, elem_res.error()};
         *ins++ = std::move(*elem_res);
     }
+
+    BOOST_JSON_INTRUSIVE_PATH_POP
+
     return result;
 }
 
@@ -339,6 +369,16 @@ value_to_impl(
     if( N != arr->size() )
     {
         BOOST_JSON_FAIL(ec, error::size_mismatch);
+        
+        using Ds = described_members<T>;
+        using D = boost::mp11::mp_first<Ds>;
+        
+        BOOST_JSON_INTRUSIVE_MESSAGE(
+            std::format(
+                "while processing array {} in {}, found {} element expected {}",
+                D::name, BOOST_JSON_INTRUSIVE::composePath(),
+                arr->size(), N));
+        
         return {boost::system::in_place_error, ec};
     }
 
@@ -373,10 +413,14 @@ struct to_described_member
                 system::error_code ec;
                 BOOST_JSON_FAIL(ec, error::size_mismatch);
                 res = {boost::system::in_place_error, ec};
+                
+                BOOST_JSON_INTRUSIVE_MESSAGE(std::format("the key >> {} << is non optional and missing in path {}", D::name, BOOST_JSON_INTRUSIVE::composePath()));
             }
             return;
         }
 
+        BOOST_JSON_INTRUSIVE_PATH_PUSH(D::name)
+
 #if defined(__GNUC__) && BOOST_GCC_VERSION >= 80000 && BOOST_GCC_VERSION < 11000
 # pragma GCC diagnostic push
 # pragma GCC diagnostic ignored "-Wunused"
@@ -386,8 +430,10 @@ struct to_described_member
 #if defined(__GNUC__) && BOOST_GCC_VERSION >= 80000 && BOOST_GCC_VERSION < 11000
 # pragma GCC diagnostic pop
 #endif
-        if( member_res )
+        if( member_res ){
             (*res).* D::pointer = std::move(*member_res);
+            BOOST_JSON_INTRUSIVE_PATH_POP
+        }
         else
             res = {boost::system::in_place_error, member_res.error()};
     }