28 Numerics library [numerics]

28.2 Numeric type requirements [numeric.requirements]

The complex and valarray components are parameterized by the type of information they contain and manipulate.
A C++ program shall instantiate these components only with a numeric type.
A numeric type is a cv-unqualified object type T that meets the Cpp17DefaultConstructible, Cpp17CopyConstructible, Cpp17CopyAssignable, and Cpp17Destructible requirements ([utility.arg.requirements]).220
If any operation on T throws an exception the effects are undefined.
In addition, many member and related functions of valarray<T> can be successfully instantiated and will exhibit well-defined behavior if and only if T meets additional requirements specified for each such member or related function.
[Example 1: 
It is valid to instantiate valarray<complex>, but operator>() will not be successfully instantiated for valarray<complex> operands, since complex does not have any ordering operators.
— end example]
In other words, value types.
These include arithmetic types, pointers, the library class complex, and instantiations of valarray for value types.