Detect std::pair non-copyability

[jagerman]

Pre-C++17, std::pair can technically have an copy constructor even
though it can't actually be invoked without a compilation failure (due
to the underlying types being non-copyable). Most stls, including
libc++ since ~3.4, use the C++17 behaviour of not exposing an uncallable
copy constructor, but FreeBSD deliberately broke their libc++ to
preserve the nonsensical behaviour
(https://svnweb.freebsd.org/base?view=revision&revision=261801).

This updates pybind's internal is_copy_constructible to also detect
the std::pair case under pre-C++17.

This also updates a few places that were using
std::is_copy_constructible to use the internal version instead.

Fixes #964

[jagerman]

I think this might not be enough. Investigating. Fixed and updated.

[rleigh-codelibre]

Thanks for looking at this so quickly!

Test results:

Standard	FreeBSD 11.0	FreeBSD 11.1
14	OK	OK
17	FAIL	OK

It now builds on FreeBSD 11.0 with C++14. However, building with -DCMAKE_CXX_STANDARD=17 fails with 11.0/clang++ 3.8.0 in the same manner as before: https://gist.github.com/rleigh-codelibre/387d25bf9da258fd80135e1a64e11335

Clang/LLVM 4.0.0 drops this option entirely, so it's now properly standards conformant from this point onward on FreeBSD.

[jagerman]

clang 3.8/C++17 is definitely unsupported (the travis-ci builds test against g++ 7 and clang/libc++ 4.0 for C++17 support). If you really need it you can try compiling with -D_LIBCPP_TRIVIAL_PAIR_COPY_CTOR=1, but realistically getting decent C++17 compiler and stl support requires a newer compiler.

[jagerman]

Also note that if you never try creating a std::pair with a non-copy-constructible type you won't actually hit the error; the test suite has various tests designed to hit various edge cases.

[rleigh-codelibre]

OK, thanks. I'll have to look at upgrading to 4.0.0 soon, so I can use C++17 without the tests failing.