12 Templates [temp]

12.4 Template constraints [temp.constr]

12.4.2 Constrained declarations [temp.constr.decl]

1
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A template declaration or function declaration can be constrained by the use of a requires-clause.
This allows the specification of constraints for that declaration as an expression:
constraint-expression:
	logical-or-expression
2
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Constraints can also be associated with a declaration through the use of constrained-parameters in a template-parameter-list.
Each of these forms introduces additional constraint-expressions that are used to constrain the declaration.
3
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A template's associated constraints are defined as follows:
The formation of the associated constraints establishes the order in which constraints are instantiated when checking for satisfaction ([temp.constr.constr]).
[ Example
: 
template<typename T> concept C = true;

template<C T> void f1(T);
template<typename T> requires C<T> void f2(T);
template<typename T> void f3(T) requires C<T>;
The functions f1, f2, and f3 have the associated constraint C<T>.
template<typename T> concept C1 = true;
template<typename T> concept C2 = sizeof(T) > 0;

template<C1 T> void f4(T) requires C2<T>;
template<typename T> requires C1<T> && C2<T> void f5(T);
The associated constraints of f4 and f5 are C1<T> ∧ C2<T>.
template<C1 T> requires C2<T> void f6();
template<C2 T> requires C1<T> void f7();
The associated constraints of f6 are C1<T> ∧ C2<T>, and those of f7 are C2<T> ∧ C1<T>.
— end example
 ]