A utility macro to expand the arguments of macro before invoking it in the preprocessor. This is mostly used to handle the __VA_ARGS__ created for variadic preprocessor macros. Often you will have to pass __VA_ARGS__ to another macro to tease out particular parameters. For example, to get the first argument, you might make something like this.
#define GET_FIRST_ARGUMENT(...) GET_FIRST_ARGUMENT_IMPL(__VA_ARGS__, no_arg)
#define GET_FIRST_ARGUMENT_IMPL(first, ...) first
You would expect this pair of macros to give you the first argument or the token no_arg if no arguments were given, and for most compilers that is what you would get. But Visual Studio in particular has a weird interpretation of the standard that causes __VA_ARGS__ to be treated as a single argument when passed to another macro. Consequently, for the example above, Visual Studio actually returns all args passed instead of the first. To get around the problem, you can wrap the entire call to the secondary macro in VTKM_EXPAND to get Visual Studio (and all other compilers) to properly treat __VA_ARGS__ as separate arguments.
#define GET_FIRST_ARGUMENT(...) VTKM_EXPAND(GET_FIRST_ARGUMENT_IMPL(__VA_ARGS__, no_arg))
#define GET_FIRST_ARGUMENT_IMPL(first, ...) first
