Remove unused would_call_std_* from NLOHMANN_CAN_CALL_STD_FUNC_IMPL

Besides detail::result_of_begin/end, which is_range and iterator_t use,
the macro defined a namespace detail2 with a tag type, a catch-all
overload and would_call_std_begin/end, plus would_call_std_begin/end
structs directly in namespace nlohmann. Nothing has used them since
they were added in #3020.

Reduce the macro to its detail part. Without the trailing struct the
';' after the two invocations would be an empty declaration that
-Wextra-semi flags, so drop it. macro_scope.hpp included
meta/detected.hpp only for this macro; all users of detected.hpp
include it (or type_traits.hpp) themselves, so remove the include.

Behavior and ABI are unchanged. The undocumented, untested and unused
names nlohmann::would_call_std_begin, nlohmann::would_call_std_end and
namespace nlohmann::detail2 are no longer declared.

Part of #5708

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

include/nlohmann/detail/macro_scope.hpp

@@ -9,7 +9,6 @@
 #pragma once
 
 #include <utility> // declval, pair
-#include <nlohmann/detail/meta/detected.hpp>
 #include <nlohmann/thirdparty/hedley/hedley.hpp>
 
 // This file contains all internal macro definitions (except those affecting ABI)
@@ -876,30 +875,6 @@ void templated_json_throw(ExceptionType exception)
     \
     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; \
-    };                                                                            \
-    } /* namespace detail2 */ \
-    \
-    template<typename... T>                                                       \
-    struct would_call_std_##std_name : detail2::would_call_std_##std_name<T...>   \
-    {                                                                             \
     }
 
 #ifndef JSON_USE_IMPLICIT_CONVERSIONS

include/nlohmann/detail/meta/call_std/begin.hpp

@@ -12,6 +12,6 @@
 
 NLOHMANN_JSON_NAMESPACE_BEGIN
 
-NLOHMANN_CAN_CALL_STD_FUNC_IMPL(begin);
+NLOHMANN_CAN_CALL_STD_FUNC_IMPL(begin)
 
 NLOHMANN_JSON_NAMESPACE_END

include/nlohmann/detail/meta/call_std/end.hpp

@@ -12,6 +12,6 @@
 
 NLOHMANN_JSON_NAMESPACE_BEGIN
 
-NLOHMANN_CAN_CALL_STD_FUNC_IMPL(end);
+NLOHMANN_CAN_CALL_STD_FUNC_IMPL(end)
 
 NLOHMANN_JSON_NAMESPACE_END