clang >=8 w/C++17 detects trying to access protected dtor

[sbchisholm]

We encountered an issue when compiling with clang-tidy >= 8.0 and C++17, constructing the boost::static_visitor, in this case count_unwrapper_t, with the initializer-list, the compiler complains about trying to access a protected destructor:

/usr/local/include/bredis/impl/protocol.ipp:292:52: error: temporary of type 'boost::static_visitor<count_variant_t<buffers_iterator<const_buffer
s_1, char>, drop_result> >' (aka 'static_visitor<variant<bredis::details::count_value_t, variant<bredis::not_enough_data_t, positive_parse_result
_t<boost::asio::buffers_iterator<boost::asio::const_buffers_1, char>, bredis::parsing_policy::drop_result>, bredis::protocol_error_t> > >') has p
rotected destructor [clang-diagnostic-error]
            boost::apply_visitor(count_unwrapper_t{}, count_result);
                                                   ^
/usr/local/include/bredis/impl/protocol.ipp:386:24: note: in instantiation of member function 'bredis::details::bulk_string_parser_t<boost::asio:
:buffers_iterator<boost::asio::const_buffers_1, char>, bredis::parsing_policy::drop_result>::apply' requested here
        return Parser::apply(next_from, to_, 1);
                       ^
...
/usr/local/include/boost/variant/static_visitor.hpp:53:5: note: declared protected here
    ~static_visitor() = default;

gcc 10.1, boost 1.69, clang 10.0, bredis 395830c

The following explanation describes how {} vs () differs in how each of them access ctor's and in our case dtor's of the parent classes:
https://stackoverflow.com/questions/56367480/should-this-code-fail-to-compile-in-c17/56367566#56367566

[codecov]

Codecov Report

Merging #41 into master will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master      #41   +/-   ##
=======================================
  Coverage   93.79%   93.79%           
=======================================
  Files          11       11           
  Lines         451      451           
=======================================
  Hits          423      423           
  Misses         28       28           

Impacted Files	Coverage Δ
include/bredis/impl/protocol.ipp	100.00% <100.00%> (ø)

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[basiliscos]

I wasn't able to reproduce it, but it seems your explanation is correct. I'll merge the PR.

Thank you for your contribution!