Fix ADL leak of nlohmann::detail through basic_json's default base class (#5238)

include/nlohmann/detail/json_custom_base_class.hpp

@@ -13,8 +13,6 @@
 #include <nlohmann/detail/abi_macros.hpp>
 
 NLOHMANN_JSON_NAMESPACE_BEGIN
-namespace detail
-{
 
 /*!
 @brief Default base class of the @ref basic_json class.
@@ -25,13 +23,27 @@ of @ref basic_json do not require complex case distinctions
 @ref basic_json always has a base class.
 By default, this class is used because it is empty and thus has no effect
 on the behavior of @ref basic_json.
+
+@note This class intentionally lives in namespace @ref nlohmann rather than
+      @ref nlohmann::detail. Every @ref basic_json specialization derives from
+      it (via @ref detail::json_base_class) unless a custom base class is
+      supplied, which makes its namespace an associated namespace of
+      @ref basic_json for the purpose of argument-dependent lookup (ADL). If
+      it lived in `nlohmann::detail`, that namespace - and with it the
+      library's internal `to_json`/`from_json` overloads - would leak into
+      ADL for any unqualified `to_json`/`from_json` call a user makes
+      involving a @ref basic_json argument, silently shadowing the user's own
+      overloads in some cases.
 */
 struct json_default_base {};
 
+namespace detail
+{
+
 template<class T>
 using json_base_class = typename std::conditional <
                         std::is_same<T, void>::value,
-                        json_default_base,
+                        ::nlohmann::json_default_base,
                         T
                         >::type;