CXX-2255 Upgrade to Ubuntu 18.04 for Evergreen tasks

[benjirewis]

CXX-2255

Upgrades Ubuntu 16.04 tasks to Ubuntu 18.04 tasks in Evergreen.

[benjirewis]

There are a number of build failures that are causing the ubuntu1804-debug-std-experimental tasks to fail. They all reference uses of scoped_bson_t. I was hoping someone might have some context on why that is.

Looks like it has to do with the overload defined for the scoped_bson_t constructor; can we just use the std library for stdx polyfills now that we've upgraded to Ubuntu 18.04?

[codecov-commenter]

Codecov Report

Merging #795 (78e94c1) into master (a2c02e6) will decrease coverage by 7.25%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #795      +/-   ##
==========================================
- Coverage   92.75%   85.50%   -7.26%     
==========================================
  Files         368      377       +9     
  Lines       21020    21757     +737     
==========================================
- Hits        19498    18603     -895     
- Misses       1522     3154    +1632     

Impacted Files	Coverage Δ
src/mongocxx/test/spec/change_stream.cpp	6.66% <0.00%> (-91.12%)	⬇️
src/mongocxx/options/private/transaction.hh	12.76% <0.00%> (-78.35%)	⬇️
src/mongocxx/options/transaction.cpp	21.21% <0.00%> (-69.70%)	⬇️
src/mongocxx/test/transactions.cpp	4.65% <0.00%> (-68.79%)	⬇️
src/mongocxx/test/change_streams.cpp	42.24% <0.00%> (-55.72%)	⬇️
src/mongocxx/exception/operation_exception.cpp	38.46% <0.00%> (-53.21%)	⬇️
src/mongocxx/test/spec/monitoring.hh	60.00% <0.00%> (-40.00%)	⬇️
src/mongocxx/options/index_view.cpp	47.61% <0.00%> (-38.10%)	⬇️
src/mongocxx/private/client_session.hh	60.95% <0.00%> (-30.97%)	⬇️
src/mongocxx/options/client_session.cpp	70.00% <0.00%> (-30.00%)	⬇️
... and 93 more

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[benjirewis]

Discussed offline: reverted the debug-std-experimental task to Ubuntu 16.04 to avoid compilation errors on g++ 5.5 (the compiler on Ubuntu 18.04) with non-standard polyfills (like EXPERIMENTAL). Filed CXX-2260 (and added a TODO) to address these errors and upgrade debug-std-experimental to Ubuntu 18.04.

[kevinAlbs]

Should this variant also be changed to test against 4.4 instead of latest in the meantime?

mongodb_version: *version_44

IIRC otherwise versioned API tests against the latest server version published on Ubuntu 16.04 may not pass.

[benjirewis]

Ah great point. Used version_44 and updated comment and display_name.

[kevinAlbs]

LGTM!

[rcsanchez97]

LGTM, module the nit on the comment.

[rcsanchez97]

I think that g++5.5 actually worked and it was something around g++7.3 and newer that gave us problems.

[benjirewis]

Ah good point. Made the comment refer generically to newer g++ versions.