Remarks: Pursuant to N4971, [namespace.std] ([spec.ext]),
users may specialize is_scalar, is_vector, and is_tensor to true
for cv-unqualified program-defined types
which respectively represent
a scalar (IEC 60050, 102-02-18),
a vector (IEC 60050, 102-03-04), and
a tensor,
and false for types which respectively do not.