<xstd/ints/memory.hpp>

namespace xstd {
  template<alignable T>
    constexpr auto align_down(T value, size_t alignment) noexcept(nothrow_alignable<T>) -> T;
  template<class T>
    auto align_down(T* pointer, size_t alignment) noexcept -> T*;
  template<alignable T>
    constexpr auto align_up(T value, size_t alignment) noexcept(nothrow_alignable<T>) -> T;
  template<class T>
    auto align_up(T* pointer, size_t alignment) noexcept -> T*;
  template<alignable T>
    constexpr auto is_aligned(T value, size_t alignment) noexcept(nothrow_alignable<T>) -> bool;
  template<class T>
    auto is_aligned(T* pointer, size_t alignment) noexcept -> bool;
}

Header	Declares
<xstd/ints/memory/align_down.hpp>	align_down
<xstd/ints/memory/align_up.hpp>	align_up
<xstd/ints/memory/is_aligned.hpp>	is_aligned

The umbrella exports the three headers under <xstd/ints/memory/>. The three agree with Boost.Align on values at every standard unsigned width and on pointers, and what align_up and align_down return satisfies is_aligned.