Let to_json(std::optional<T>) propagate exceptions from T's to_json

The overload was marked noexcept even though its body assigns *opt to
the JSON value, which calls T's to_json (or allocates for std::string,
std::vector, or json). Any exception from there -- a user-defined
to_json reporting an error, or std::bad_alloc -- called std::terminate()
instead of propagating. The noexcept also made basic_json's converting
constructor noexcept(true) for std::optional<T>, so json j = opt; could
not report the error either.

Fixes #5642.
Signed-off-by: Niels Lohmann <mail@nlohmann.me>

single_include/nlohmann/json.hpp

@@ -6816,7 +6816,7 @@ struct external_constructor<value_t::object>
 #ifdef JSON_HAS_CPP_17
 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
+void to_json(BasicJsonType& j, const std::optional<T>& opt)
 {
     if (opt.has_value())
     {