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Add JSON_DISABLE_TUPLE_REFERENCE_CONVERSION to fix std::tuple conversions (#5598)

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* Add JSON_DISABLE_TUPLE_REFERENCE_CONVERSION to fix std::tuple conversions

basic_json can be constructed from std::tuple<json&>, which it turns into
a one-element array. Because of this, std::tuple picks its converting
constructor that converts the whole source tuple instead of the
element-wise one. As a result, std::tuple<const json&> built from
std::forward_as_tuple(j) binds to a temporary (a compile error with libc++,
a dangling reference with other standard libraries), and std::tuple<json>
built the same way holds [j] instead of a copy of j.

The new opt-in macro JSON_DISABLE_TUPLE_REFERENCE_CONVERSION (CMake option
JSON_DisableTupleReferenceConversion) removes the conversion from a
one-element tuple holding a reference to the same basic_json type, so
std::tuple converts element-wise. It is off by default, so existing
behavior is unchanged. It does not change any function body and therefore
is not part of the ABI tag.

Fixes #2226

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

* Convert one-element tuples to arrays on every compiler

to_json for std::tuple assigns a braced list, j = { std::get<Idx>(t)... }.
With a single element that is itself a basic_json, Apple clang 15 and 16
treat j = {x} as a copy of x, so std::tuple<json>{true} became true
instead of [true]. The macOS jobs (Xcode 15.1, 16.1) failed the new
checks in unit-disable-tuple-reference-conversion and unit-regression2.

The one-element overload that already handles
JSON_BRACE_INIT_COPY_SEMANTICS builds the array (or object, for a
[string, value] element) explicitly, the same way the initializer-list
constructor does. Use it unconditionally. The output is unchanged on
compilers that already wrapped the element.

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

* Skip json reference tuple tests on clang < 4 and GCC < 5

ci_test_compilers_gcc_old (4.8) and ci_test_compilers_clang (3.4) could
not compile the new tuple tests. Creating a std::tuple of basic_json
references, e.g. std::forward_as_tuple(j), makes these compilers
instantiate basic_json's conversion operator for libstdc++'s internal
tuple bases, which fails hard. This happens with and without
JSON_DISABLE_TUPLE_REFERENCE_CONVERSION, so it is a limitation of these
compilers, not of the new option.

Tested with the CI images: clang 3.4 to 3.9 and GCC 4.8 and 4.9 fail,
clang 4, 5, and 6 and GCC 5 and 6 compile all cases. Skip only the
checks that create such tuples; the is_constructible checks still run.

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

---------

Signed-off-by: Niels Lohmann <mail@nlohmann.me>
This commit is contained in:
Niels Lohmann
2026-09-30 20:53:56 +02:00
committed by GitHub
parent 6a073dbae4
commit 3b6ae43c53
18 changed files with 334 additions and 15 deletions
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single_include/nlohmann/json.hpp
+30 -7
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@@ -3329,6 +3329,10 @@ void templated_json_throw(ExceptionType exception)
#define JSON_DISABLE_ENUM_SERIALIZATION 0
#endif
#ifndef JSON_DISABLE_TUPLE_REFERENCE_CONVERSION
#define JSON_DISABLE_TUPLE_REFERENCE_CONVERSION 0
#endif
#if JSON_HAS_THREE_WAY_COMPARISON
#include <compare> // partial_ordering
#endif
@@ -4644,6 +4648,18 @@ template<typename BasicJsonType, typename CompatibleType>
struct is_compatible_type
: is_compatible_type_impl<BasicJsonType, CompatibleType> {};
// a one-element std::tuple holding a reference to BasicJsonType, as created by
// std::forward_as_tuple(j); see JSON_DISABLE_TUPLE_REFERENCE_CONVERSION
template<typename BasicJsonType, typename T>
struct is_basic_json_reference_tuple : std::false_type {};
template<typename BasicJsonType, typename T>
struct is_basic_json_reference_tuple<BasicJsonType, std::tuple<T>>
{
static constexpr bool value =
std::is_reference<T>::value && std::is_same<uncvref_t<T>, BasicJsonType>::value;
};
template<typename BasicJsonType, typename CompatibleArrayType>
struct is_compatible_binary_type
{
@@ -7002,11 +7018,13 @@ inline void to_json_tuple_impl(BasicJsonType& j, const Tuple& t, index_sequence<
j = { std::get<Idx>(t)... };
}
#if JSON_BRACE_INIT_COPY_SEMANTICS
// JSON_BRACE_INIT_COPY_SEMANTICS makes a one-element braced list copy its
// element instead of wrapping it, which would serialize std::tuple<int>{5} as 5
// rather than [5]. Build what the default deduction builds instead: an object
// if the element is a [string, value] pair, a one-element array otherwise.
// A one-element braced list does not reliably wrap its element: with
// JSON_BRACE_INIT_COPY_SEMANTICS it copies it, which would serialize
// std::tuple<int>{5} as 5 rather than [5], and some compilers (e.g., Apple clang
// 15 and 16) copy an element that is itself a basic_json even without it, so
// std::tuple<json>{true} became true rather than [true]. Build what the default
// deduction builds instead: an object if the element is a [string, value] pair,
// a one-element array otherwise.
template<typename BasicJsonType, typename Tuple>
inline void to_json_tuple_impl(BasicJsonType& j, const Tuple& t, index_sequence<0> /*unused*/)
{
@@ -7024,7 +7042,6 @@ inline void to_json_tuple_impl(BasicJsonType& j, const Tuple& t, index_sequence<
j = BasicJsonType::array({std::move(element)});
}
}
#endif
template<typename BasicJsonType, typename Tuple>
inline void to_json_tuple_impl(BasicJsonType& j, const Tuple& /*unused*/, index_sequence<> /*unused*/)
@@ -28523,7 +28540,12 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
template < typename CompatibleType,
typename U = detail::uncvref_t<CompatibleType>,
detail::enable_if_t <
!detail::is_basic_json<U>::value && detail::is_compatible_type<basic_json_t, U>::value, int > = 0 >
!detail::is_basic_json<U>::value && detail::is_compatible_type<basic_json_t, U>::value
#if JSON_DISABLE_TUPLE_REFERENCE_CONVERSION
// see https://github.com/nlohmann/json/issues/2226
&& !detail::is_basic_json_reference_tuple<basic_json_t, U>::value
#endif
, int > = 0 >
basic_json(CompatibleType && val) noexcept(noexcept( // NOLINT(bugprone-forwarding-reference-overload,bugprone-exception-escape)
JSONSerializer<U>::to_json(std::declval<basic_json_t&>(),
std::forward<CompatibleType>(val))))
@@ -33572,6 +33594,7 @@ struct formatter<nlohmann::NLOHMANN_BASIC_JSON_TPL, char> // NOLINT(cert-dcl58-c
#undef JSON_INLINE_VARIABLE
#undef JSON_NO_UNIQUE_ADDRESS
#undef JSON_DISABLE_ENUM_SERIALIZATION
#undef JSON_DISABLE_TUPLE_REFERENCE_CONVERSION
#ifndef JSON_TEST_KEEP_MACROS
#undef JSON_CATCH