Properly constrain the basic_json conversion operator

Fixes #2491

include/nlohmann/detail/meta/detected.hpp

@@ -39,6 +39,9 @@ struct detector<Default, void_t<Op<Args...>>, Op, Args...>
 template<template<class...> class Op, class... Args>
 using is_detected = typename detector<nonesuch, void, Op, Args...>::value_t;
 
+template<template<class...> class Op, class... Args>
+struct is_detected_lazy : is_detected<Op, Args...> { };
+
 template<template<class...> class Op, class... Args>
 using detected_t = typename detector<nonesuch, void, Op, Args...>::type;
 

include/nlohmann/detail/meta/type_traits.hpp

@@ -157,6 +157,9 @@ template<class B1, class... Bn>
 struct conjunction<B1, Bn...>
 : std::conditional<bool(B1::value), conjunction<Bn...>, B1>::type {};
 
+// https://en.cppreference.com/w/cpp/types/negation
+template<class B> struct negation : std::integral_constant < bool, !B::value > { };
+
 // Reimplementation of is_constructible and is_default_constructible, due to them being broken for
 // std::pair and std::tuple until LWG 2367 fix (see https://cplusplus.github.io/LWG/lwg-defects.html#2367).
 // This causes compile errors in e.g. clang 3.5 or gcc 4.9.

include/nlohmann/json.hpp

@@ -3359,17 +3359,19 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
     @since version 1.0.0
     */
     template < typename ValueType, typename std::enable_if <
-                   !std::is_pointer<ValueType>::value&&
-                   !std::is_same<ValueType, detail::json_ref<basic_json>>::value&&
-                   !std::is_same<ValueType, typename string_t::value_type>::value&&
-                   !detail::is_basic_json<ValueType>::value
-                   && !std::is_same<ValueType, std::initializer_list<typename string_t::value_type>>::value
+                   detail::conjunction <
+                       detail::negation<std::is_pointer<ValueType>>,
+                       detail::negation<std::is_same<ValueType, detail::json_ref<basic_json>>>,
+                                        detail::negation<std::is_same<ValueType, typename string_t::value_type>>,
+                                        detail::negation<detail::is_basic_json<ValueType>>,
+                                        detail::negation<std::is_same<ValueType, std::initializer_list<typename string_t::value_type>>>,
+
 #if defined(JSON_HAS_CPP_17) && (defined(__GNUC__) || (defined(_MSC_VER) && _MSC_VER >= 1910 && _MSC_VER <= 1914))
-                   && !std::is_same<ValueType, typename std::string_view>::value
+                                                detail::negation<std::is_same<ValueType, std::string_view>>,
 #endif
-                   && detail::is_detected<detail::get_template_function, const basic_json_t&, ValueType>::value
-                   , int >::type = 0 >
-    JSON_EXPLICIT operator ValueType() const
+                                                detail::is_detected_lazy<detail::get_template_function, const basic_json_t&, ValueType>
+                                                >::value, int >::type = 0 >
+                                        JSON_EXPLICIT operator ValueType() const
     {
         // delegate the call to get<>() const
         return get<ValueType>();
@@ -8907,7 +8909,7 @@ template<>
 inline void swap<nlohmann::json>(nlohmann::json& j1, nlohmann::json& j2) noexcept( // NOLINT(readability-inconsistent-declaration-parameter-name)
     is_nothrow_move_constructible<nlohmann::json>::value&&  // NOLINT(misc-redundant-expression)
     is_nothrow_move_assignable<nlohmann::json>::value
-)
+                              )
 {
     j1.swap(j2);
 }

single_include/nlohmann/json.hpp

@@ -3309,6 +3309,9 @@ struct detector<Default, void_t<Op<Args...>>, Op, Args...>
 template<template<class...> class Op, class... Args>
 using is_detected = typename detector<nonesuch, void, Op, Args...>::value_t;
 
+template<template<class...> class Op, class... Args>
+struct is_detected_lazy : is_detected<Op, Args...> { };
+
 template<template<class...> class Op, class... Args>
 using detected_t = typename detector<nonesuch, void, Op, Args...>::type;
 
@@ -3554,6 +3557,9 @@ template<class B1, class... Bn>
 struct conjunction<B1, Bn...>
 : std::conditional<bool(B1::value), conjunction<Bn...>, B1>::type {};
 
+// https://en.cppreference.com/w/cpp/types/negation
+template<class B> struct negation : std::integral_constant < bool, !B::value > { };
+
 // Reimplementation of is_constructible and is_default_constructible, due to them being broken for
 // std::pair and std::tuple until LWG 2367 fix (see https://cplusplus.github.io/LWG/lwg-defects.html#2367).
 // This causes compile errors in e.g. clang 3.5 or gcc 4.9.
@@ -20422,17 +20428,19 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
     @since version 1.0.0
     */
     template < typename ValueType, typename std::enable_if <
-                   !std::is_pointer<ValueType>::value&&
-                   !std::is_same<ValueType, detail::json_ref<basic_json>>::value&&
-                   !std::is_same<ValueType, typename string_t::value_type>::value&&
-                   !detail::is_basic_json<ValueType>::value
-                   && !std::is_same<ValueType, std::initializer_list<typename string_t::value_type>>::value
+                   detail::conjunction <
+                       detail::negation<std::is_pointer<ValueType>>,
+                       detail::negation<std::is_same<ValueType, detail::json_ref<basic_json>>>,
+                                        detail::negation<std::is_same<ValueType, typename string_t::value_type>>,
+                                        detail::negation<detail::is_basic_json<ValueType>>,
+                                        detail::negation<std::is_same<ValueType, std::initializer_list<typename string_t::value_type>>>,
+
 #if defined(JSON_HAS_CPP_17) && (defined(__GNUC__) || (defined(_MSC_VER) && _MSC_VER >= 1910 && _MSC_VER <= 1914))
-                   && !std::is_same<ValueType, typename std::string_view>::value
+                                                detail::negation<std::is_same<ValueType, std::string_view>>,
 #endif
-                   && detail::is_detected<detail::get_template_function, const basic_json_t&, ValueType>::value
-                   , int >::type = 0 >
-    JSON_EXPLICIT operator ValueType() const
+                                                detail::is_detected_lazy<detail::get_template_function, const basic_json_t&, ValueType>
+                                                >::value, int >::type = 0 >
+                                        JSON_EXPLICIT operator ValueType() const
     {
         // delegate the call to get<>() const
         return get<ValueType>();

test/src/unit-regression2.cpp

@@ -39,6 +39,7 @@ using nlohmann::json;
 
 #include <list>
 #include <cstdio>
+#include <type_traits>
 #include <utility>
 
 #if (defined(__cplusplus) && __cplusplus >= 201703L) || (defined(_HAS_CXX17) && _HAS_CXX17 == 1) // fix for issue #464
@@ -620,4 +621,9 @@ TEST_CASE("regression tests 2")
             nlohmann::to_json(o["foo"], s);
         }
     }
+
+    SECTION("issue #2825 - Properly constrain the basic_json conversion operator")
+    {
+        static_assert(std::is_copy_assignable<nlohmann::ordered_json>::value, "");
+    }
 }