unspecified2 is a
type that meets the requirements of a constant
Cpp17RandomAccessIterator
and models contiguous_iterator,
whose value_type is the template
parameter T and whose reference type is const T&.
The iterators returned by begin and end for an array
are guaranteed to be valid until the member function
resize(size_t, T) is called for that
array or until the lifetime of that array ends, whichever happens
first.