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Fix std::terminate when converting std::optional with MSVC 2017

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MSVC 2017 evaluates std::is_nothrow_assignable<json&, const T&> as true
even if T's to_json throws, so to_json(json&, const std::optional<T>&)
was noexcept there and the exception from #5642's test called
std::terminate instead of propagating. Make that conversion never
noexcept on MSVC 2017; all other compilers keep the exact condition.
The static_asserts on the condition are skipped for MSVC 2017; the
runtime check that the exception propagates still runs there.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>
This commit is contained in:
Niels Lohmann committed 2026-10-05 20:08:17 +02:00
1 parent 8c05e40cae
commit 2234cd2d40
3 files changed
+27 -5
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include/nlohmann/detail/conversions/to_json.hpp
+12 -1
View File
@@ -287,9 +287,20 @@ struct external_constructor<value_t::object>
/////////////
#ifdef JSON_HAS_CPP_17
// whether storing the value of a std::optional<T> cannot throw; MSVC 2017
// evaluates std::is_nothrow_assignable as true even if T's to_json throws, so
// the exception would call std::terminate (#5642)
#if defined(_MSC_VER) && !defined(__clang__) && _MSC_VER < 1920
template<typename BasicJsonType, typename T>
using is_nothrow_optional_to_json = std::false_type;
#else
template<typename BasicJsonType, typename T>
using is_nothrow_optional_to_json = std::is_nothrow_assignable<BasicJsonType&, const T&>;
#endif
template<typename BasicJsonType, typename T,
enable_if_t<std::is_constructible<BasicJsonType, T>::value, int> = 0>
void to_json(BasicJsonType& j, const std::optional<T>& opt) noexcept(std::is_nothrow_assignable<BasicJsonType&, const T&>::value)
void to_json(BasicJsonType& j, const std::optional<T>& opt) noexcept(is_nothrow_optional_to_json<BasicJsonType, T>::value)
{
if (opt.has_value())
{