13 Templates [temp]

13.5 Template constraints [temp.constr]

13.5.3 Constrained declarations [temp.constr.decl]

A template declaration ([temp.pre]) or templated function declaration ([dcl.fct]) 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

Constraints can also be associated with a declaration through the use of type-constraints in a template-parameter-list or parameter-type-list.

Each of these forms introduces additional constraint-expressions that are used to constrain the declaration.

A declaration's associated constraints are defined as follows:

(3.1)
If there are no introduced constraint-expressions, the declaration has no associated constraints.
(3.2)
Otherwise, if there is a single introduced constraint-expression, the associated constraints are the normal form of that expression.
(3.3)
Otherwise, the associated constraints are the normal form of a logical and expression whose operands are in the following order:
- (3.3.1)
  the constraint-expression introduced by each type-constraint ([temp.param]) in the declaration's template-parameter-list, in order of appearance, and
- (3.3.2)
  the constraint-expression introduced by a requires-clause following a template-parameter-list ([temp.pre]), and
- (3.3.3)
  the constraint-expression introduced by each type-constraint in the parameter-type-list of a function declaration, and
- (3.3.4)
  the constraint-expression introduced by a trailing requires-clause ([dcl.decl]) of a function declaration ([dcl.fct]).

The formation of the associated constraints establishes the order in which constraints are instantiated when checking for satisfaction ([temp.constr.constr]).

[Example 1: 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]

When determining whether a given introduced constraint-expression

C_{1}

of a declaration in an instantiated specialization of a templated class is equivalent ([temp.over.link]) to the corresponding constraint-expression

C_{2}

of a declaration outside the class body,

C_{1}

is instantiated.

If the instantiation results in an invalid expression, the constraint-expressions are not equivalent.

[Note 1:

This can happen when determining which member template is specialized by an explicit specialization declaration.

— end note]

[Example 2: template <class T> concept C = true; template <class T> struct A { template <class U> U f(U) requires C<typename T::type>; // #1 template <class U> U f(U) requires C<T>; // #2 }; template <> template <class U> U A<int>::f(U u) requires C<int> { return u; } // OK, specializes #2

Substituting int for T in C<typename T::type> produces an invalid expression, so the specialization does not match #1.

Substituting int for T in C<T> produces C<int>, which is equivalent to the constraint-expression for the specialization, so it does match #2.

— end example]