There was a legitimate reason to have this duplication when the library
was much smaller. However, hovering at about 30 kLOCs, the library is
definitely not small enough anymore to mandate reimplementing our own
<type_traits>. Also, most user code will probably include <type_traits>
anyway, so using <type_traits> will not increase the overall compilation
time in actual code bases.
Finally, this gives us much more flexibility, like the ability to
inherit our IntegralConstant from std::integral_constant, and then
return that from the traits:: metafunctions, so that operators can
be used with them.
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