Currently, tests are scattered throughout multiple directories, making
it difficult to determine what is tested and what is not tested.
With upcoming work on interoperability between Swift and C++
(https://forums.swift.org/t/manifesto-interoperability-between-swift-and-c/33874),
we'd like to rearrange the tests and bring tests covering a single C/C++
feature next to each other.
This PR also adds tests for enabling C++ interop where possible.
This is a new feature of Swift 5 mode, so it deserves at least a
little bit of explanation right in the diagnostic. If you have an
otherwise-fully-covered switch but can't assume the enum is frozen,
you'll now get this message:
switch covers known cases, but 'MusicGenre' may have additional
unknown values
Furthermore, if the enum comes from a system header, it looks like
this:
switch covers known cases, but 'NSMusicGenre' may have additional
unknown values, possibly added in future versions
...to further suggest the idea that even though your switch is covered
/now/, it might not handle everything in the /future/. This extra bit
is limited to system headers to avoid showing up on C enums defined in
your own project, for which it sounds silly. (The main message is
still valid though, since you can cram whatever you want into a C
enum, and people use this pattern to implement "private cases".)
rdar://problem/39367045
