Although `vtkm::internal::Variant` respected the trivially copyable
attribute of the types it contains, it was never totally trivial (i.e.
`std::is_trivial<Variant<...>>` was never true). The reason was that
`Variant` was initializing its `Index` parameter to signify that it was
not initialized. However, the fact that `Index` was initialized meant
that it was not trivially constructed.
Now, `Variant` type checks its types to see if they are all trivially
constructible. If so, it makes itself trivially constructible.
This means that `Index` may or may not be valid if `Variant` is
constructed without an argument. This in turn means that the result of
`Variant::IsValid` becomes undefined. That should be OK in practice.
`Index` will "point" to an uninitialized object, but that object is
trivially constructed anyway. However, that could cause problems if
developers used `IsValid` to determine if something is selected.
`integer_sequence` is an essential tool when dealing with structures
like a `Tuple`. Make sure that we have one (even though it was not
introduced until C++14).
Currently, VTK-m is using C++11. However, it is often useful to use
features in the `std` namespace that are defined for C++14 or later. We
can provide our own versions (sometimes), but it is preferable to use
the version provided by the compiler if available.
There were already some examples of defining portable versions of C++14
and C++17 classes in a `vtkmstd` namespace, but these were sprinkled
around the source code.
There is now a top level `vtkmstd` directory and in it are header files
that provide portable versions of these future C++ classes. In each
case, preprocessor macros are used to select which version of the class
to use.
