13 Templates [temp]

13.5 Template constraints [temp.constr]

13.5.2 Constraints [temp.constr.constr] General [temp.constr.constr.general]

A constraint is a sequence of logical operations and operands that specifies requirements on template arguments.
The operands of a logical operation are constraints.
There are four different kinds of constraints:
In order for a constrained template to be instantiated ([temp.spec]), its associated constraints shall be satisfied as described in the following subclauses.
[Note 1: 
Forming the name of a specialization of a class template, a variable template, or an alias template ([temp.names]) requires the satisfaction of its constraints.
Overload resolution requires the satisfaction of constraints on functions and function templates.
— end note]