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Fix object_comparator_t for object types without key_compare

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detail::actual_object_comparator selected between object_t::key_compare and
default_object_comparator_t with std::conditional. Both type arguments of
std::conditional are named eagerly, so object_t::key_compare had to exist
regardless of the condition, and the has_key_compare guard added in 3.11.0
never took effect: any ObjectType without a key_compare member type failed to
compile while instantiating basic_json itself.

Use detected_or_t instead, which resolves through a SFINAE partial
specialization and only names object_t::key_compare when it exists. The
selected type is unchanged for every object type that compiled before, so
object_comparator_t -- a public member type -- keeps its meaning and ABI.

has_key_compare had no other users and is removed.

Add a regression test using an adapter around std::unordered_map, which has no
key_compare; it fails to compile without this change.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_018hxZxz8svM54c6ATEvXp5E
Signed-off-by: Niels Lohmann <mail@nlohmann.me>
This commit is contained in:
Niels Lohmann
2026-08-28 17:38:55 +00:00
co-authored by Claude Opus 5
parent 599bb1b68c
commit b1c9a68b9b
3 changed files with 116 additions and 12 deletions
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include/nlohmann/detail/meta/type_traits.hpp
+7 -6
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@@ -172,17 +172,18 @@ struct has_to_json < BasicJsonType, T, enable_if_t < !is_basic_json<T>::value >>
template<typename T>
using detect_key_compare = typename T::key_compare;
template<typename T>
struct has_key_compare : std::integral_constant<bool, is_detected<detect_key_compare, T>::value> {};
// obtains the actual object key comparator
// obtains the actual object key comparator: object_t::key_compare if the
// object type defines it, and default_object_comparator_t otherwise
//
// note detected_or_t is used rather than std::conditional, because the latter
// names both of its type arguments eagerly; object_t::key_compare would then
// be a hard error for an object type that does not define it
template<typename BasicJsonType>
struct actual_object_comparator
{
using object_t = typename BasicJsonType::object_t;
using object_comparator_t = typename BasicJsonType::default_object_comparator_t;
using type = typename std::conditional < has_key_compare<object_t>::value,
typename object_t::key_compare, object_comparator_t>::type;
using type = detected_or_t<object_comparator_t, detect_key_compare, object_t>;
};
template<typename BasicJsonType>