meta: fix is_compatible/constructible traits

The previous version relied on the existence of an 'iterator' type. As mentioned in comments, this is not the proper way to do it and causes issues with certain types (e.g. views from range-v3). Add a 'is_range' trait that properly detects the return type of 'begin'/'end', and use it in instead.
nlohmann · Sep 17, 2021 · 7fb7fc8 · 7fb7fc8
1 parent 0b345b2
commit 7fb7fc8
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Showing 9 changed files with 792 additions and 416 deletions.
diff --git a/include/nlohmann/detail/macro_scope.hpp b/include/nlohmann/detail/macro_scope.hpp
@@ -1,7 +1,8 @@
 #pragma once
 
-#include <utility> // pair
+#include <utility> // declval, pair
 #include <nlohmann/thirdparty/hedley/hedley.hpp>
+#include <nlohmann/detail/meta/detected.hpp>
 
 // This file contains all internal macro definitions
 // You MUST include macro_unscope.hpp at the end of json.hpp to undef all of them
@@ -292,6 +293,41 @@
     inline void to_json(nlohmann::json& nlohmann_json_j, const Type& nlohmann_json_t) { NLOHMANN_JSON_EXPAND(NLOHMANN_JSON_PASTE(NLOHMANN_JSON_TO, __VA_ARGS__)) } \
     inline void from_json(const nlohmann::json& nlohmann_json_j, Type& nlohmann_json_t) { NLOHMANN_JSON_EXPAND(NLOHMANN_JSON_PASTE(NLOHMANN_JSON_FROM, __VA_ARGS__)) }
 
+
+// source: https://stackoverflow.com/a/26745591
+
+#define NLOHMANN_CAN_CALL_STD_FUNC_IMPL(std_name)                                 \
+    namespace detail {                                                            \
+    using std::std_name;                                                          \
+    \
+    template<typename... T>                                                       \
+    using result_of_##std_name = decltype(std_name(std::declval<T>()...));        \
+    }                                                                             \
+    \
+    namespace detail2 {                                                           \
+    struct std_name##_tag                                                         \
+    {                                                                             \
+    };                                                                            \
+    \
+    template<typename... T>                                                       \
+    std_name##_tag std_name(T&&...);                                              \
+    \
+    template<typename... T>                                                       \
+    using result_of_##std_name = decltype(std_name(std::declval<T>()...));        \
+    \
+    template<typename... T>                                                       \
+    struct would_call_std_##std_name                                              \
+    {                                                                             \
+        static constexpr auto const value = ::nlohmann::detail::                  \
+                                            is_detected_exact<std_name##_tag, result_of_##std_name, T...>::value; \
+    };                                                                            \
+    }                                                                             \
+    \
+    template<typename... T>                                                       \
+    struct would_call_std_##std_name : detail2::would_call_std_##std_name<T...>   \
+    {                                                                             \
+    }
+
 #ifndef JSON_USE_IMPLICIT_CONVERSIONS
     #define JSON_USE_IMPLICIT_CONVERSIONS 1
 #endif

diff --git a/include/nlohmann/detail/macro_unscope.hpp b/include/nlohmann/detail/macro_unscope.hpp
@@ -19,5 +19,7 @@
 #undef NLOHMANN_BASIC_JSON_TPL_DECLARATION
 #undef NLOHMANN_BASIC_JSON_TPL
 #undef JSON_EXPLICIT
+#undef JSON_CATCH
+#undef NLOHMANN_CAN_CALL_STD_FUNC_IMPL
 
 #include <nlohmann/thirdparty/hedley/hedley_undef.hpp>
diff --git a/include/nlohmann/detail/meta/call_std/begin.hpp b/include/nlohmann/detail/meta/call_std/begin.hpp
@@ -0,0 +1,8 @@
+#pragma once
+
+#include <nlohmann/detail/meta/can_call_std.hpp>
+
+namespace nlohmann
+{
+NLOHMANN_CAN_CALL_STD_FUNC_IMPL(begin);
+}
diff --git a/include/nlohmann/detail/meta/call_std/end.hpp b/include/nlohmann/detail/meta/call_std/end.hpp
@@ -0,0 +1,8 @@
+#pragma once
+
+#include <nlohmann/detail/meta/can_call_std.hpp>
+
+namespace nlohmann
+{
+NLOHMANN_CAN_CALL_STD_FUNC_IMPL(end);
+}
diff --git a/include/nlohmann/detail/meta/can_call_std.hpp b/include/nlohmann/detail/meta/can_call_std.hpp
@@ -0,0 +1,37 @@
+#pragma once
+
+#include <nlohmann/detail/meta/detected.hpp>
+
+// source: https://stackoverflow.com/a/26745591
+
+#define CAN_CALL_STD_FUNC_IMPL(std_name)                                          \
+    namespace detail {                                                            \
+    using std::std_name;                                                          \
+    \
+    template<typename... T>                                                       \
+    using result_of_##std_name = decltype(std_name(std::declval<T>()...));        \
+    }                                                                             \
+    \
+    namespace detail2 {                                                           \
+    struct std_name##_tag                                                         \
+    {                                                                             \
+    };                                                                            \
+    \
+    template<typename... T>                                                       \
+    std_name##_tag std_name(T&&...);                                              \
+    \
+    template<typename... T>                                                       \
+    using result_of_##std_name = decltype(std_name(std::declval<T>()...));        \
+    \
+    template<typename... T>                                                       \
+    struct would_call_std_##std_name                                              \
+    {                                                                             \
+        static constexpr auto const value = ::nlohmann::detail::                  \
+                                            is_detected_exact<std_name##_tag, result_of_##std_name, T...>::value; \
+    };                                                                            \
+    }                                                                             \
+    \
+    template<typename... T>                                                       \
+    struct would_call_std_##std_name : detail2::would_call_std_##std_name<T...>   \
+    {                                                                             \
+    }