Classify leaves with operator<=> itself past the nesting bound

mirror of https://github.com/nlohmann/json.git synced 2026-10-03 05:00:30 +00:00

In C++20, an ordered comparison past the nesting bound classified a pair of
leaves by asking == first and then order_leaves(), which calls < and > -
both derived from <=>. For a pair of binary values with the same bytes but a
different subtype, == reports them unequal, while <=> (through
std::vector<std::uint8_t>::operator<=>) reports them equivalent, so the pair
ended the comparison as unordered instead of letting the next element
decide - unlike an array or object within the bound, which compares such a
pair with its own operator<=> and gets equivalent. So operator<=>, and the
<, <=, >, >= derived from it, could give a different result for the same two
values depending on how deeply the values were nested, or unordered at every
depth with JSON_NO_THREAD_LOCAL defined.

compare_leaves() now classifies such a pair in C++20 with operator<=> itself
instead, matching how a value within the bound is compared; the equality-only
and pre-C++20 ordered cases are unchanged. Which of the three runs is chosen
by overloading on std::integral_constant<bool, Ordered>, the same tag
dispatch order_leaves() already uses, rather than a runtime "if (Ordered)" on
a template parameter, which MSVC would flag as a constant condition (C4127).

Added a regression test to unit-comparison.cpp that nests such a pair 0, 127,
128 and 200 levels deep (127 stays within the 128-level bound, 128 and 200
do not) and checks that operator<=> and operator< agree at every depth.

Fixes #5654.

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

This commit is contained in:

Niels Lohmann

2026-09-29 23:20:26 +02:00

parent 633de8e44b

commit 3d3b90b05d

4 changed files with 154 additions and 3 deletions

									
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				## Version history

				1. Added in version 3.11.0.

				1. Added in version 3.11.0. Fixed in version 3.13.0 to keep the result independent of nesting depth: before, a pair

				   of binary values with the same bytes but a different subtype - unequal by `operator==`, yet equivalent by

				   `operator<=>` - could end an ordered comparison as `unordered` only past 128 levels of nesting, or at every depth

				   with [`JSON_NO_THREAD_LOCAL`](../macros/json_no_thread_local.md) defined.

				2. Added in version 3.11.0.