from_json() for C arrays had four hand-unrolled overloads (rank 1-4,
added incrementally in #4262), each with its own nested loops. to_json()
already handles any rank recursively, so a rank-5+ C array could be
serialized but not read back with get_to()/get<>().
Replace the four overloads with one from_json() SFINAE-constrained on
get<remove_all_extents<T>::type>() existing, forwarding to a pair of
mutually recursive from_json_c_array_element() helpers: one assigns a
non-array element via get<T>(), the other loops over a array element and
recurses one dimension at a time. Each dimension still goes through at(),
so type_error.304/out_of_range.401 stay unchanged; ranks 1-4 keep their
existing behavior and semantics.
Adds rank-5 round-trip and mismatched-shape tests to unit-conversions.cpp.
Public API: additive only (rank 5+ C arrays become readable).
Signed-off-by: Niels Lohmann <mail@nlohmann.me>
#5708 item 1
Niels Lohmann
23aa069e63