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Properly constrain the basic_json conversion operator

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Fixes #2491
This commit is contained in:
Louis Dionne
2021-06-17 13:17:43 -04:00
parent 926fab47d0
commit b0e5965d71
5 changed files with 41 additions and 19 deletions
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26
single_include/nlohmann/json.hpp
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@ -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>();