With GCC 10 and older in C++11/14 mode, a free to_json() for a type
holding an optional<Dummy> (Dummy constructible from json) fails with
"invalid use of incomplete type detector<...to_json_function...>".
ADL for Dummy finds the unrelated to_json and closes an instantiation
cycle through optional's converting constructor. The same error
reproduces without the library, so it can't be fixed here.
Add a FAQ entry explaining the cause and the hidden-friend workaround,
recommend hidden friends in the arbitrary types docs, and add a
regression test that keeps the workaround compiling on GCC 7-10.
Signed-off-by: Niels Lohmann <mail@nlohmann.me>
Niels Lohmann
86ea63185e