When converting objects or strings between different basic_json specializations,
the target's object_t::key_type or string_t must be directly constructible from
the source's corresponding type. If this requirement is not met, the conversion
silently falls back to the array-conversion path, producing incorrect results.
This documents the limitation and provides references to issue #3425, which tracks
this behavior. The comment in unit-alt-string.cpp is clarified to reference the
known limitation with a link to the issue, and suggests the parse() workaround.
Fixes#3425 (documentation; full fix deferred pending type-trait redesign)
Signed-off-by: Niels Lohmann <mail@nlohmann.me>
Niels Lohmann
fe0299545a