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2ad532df43
we format `std::variant<std::monostate, seastar::timed_out_error,
raft::not_a_leader, raft::dropped_entry, raft::commit_status_unknown,
raft::conf_change_in_progress, raft::stopped_error, raft::not_a_member>`
in this source file. and currently, we format `std::variant<..>` using
the default-generated `fmt::formatter` from `operator<<`, so in order to
format it using {fmt}'s compile-time check enabled, we have to make the
`operator<<` overload for `std::variant<...>` visible from the caller
sites which format `std::variant<...>` using {fmt}.
in this change, the `operator<<` for `std::variant<...>` is moved to
from the middle of the source file to the top of it, so that it can
be found when the compiler looks up for a matched `fmt::formatter`
for `std::variant<...>`.
please note, we cannot use the `fmt::formatter` provided by `fmt/std.h`,
as its specialization for `std::variant` requires that all the types
of the variant is `is_formattable`. but the default generated formatter
for type `T` is not considered as the proof that `T` is formattable.
this should address the FTBFS with the latest seastar like:
```
/usr/include/fmt/core.h:2743:12: error: call to deleted constructor of 'conditional_t<has_formatter<mapped_type, context>::value, formatter<mapped_type, char_type>, fallback_formatter<stripped_type, char_type>>' (aka 'fmt::detail::fallback_formatter<std::variant<std::monostate, seastar::timed_out_error, raft::not_a_leader, raft::dropped_entry, raft::commit_status_unknown, raft::conf_change_in_progress, raft::stopped_error, raft::not_a_member>>')
```
Signed-off-by: Kefu Chai <kefu.chai@scylladb.com>
Closes scylladb/scylladb#16616