13 Templates [temp]

13.5 Template constraints [temp.constr]

13.5.2 Constraints [temp.constr.constr]

13.5.2.4 Concept-dependent constraints [temp.constr.concept]

A concept-dependent constraint CD is an atomic constraint whose expression is a concept-id CI whose concept-name names a dependent concept named C.

To determine if CD is satisfied, the parameter mapping and template arguments are first substituted into C.

If substitution results in an invalid concept-id in the immediate context of the constraint ([temp.deduct.general]), the constraint is not satisfied.

Otherwise, let CI

^{'}

be the normal form ([temp.constr.normal]) of the concept-id after substitution of C.

[Note 1:

Normalization of CI might be ill-formed; no diagnostics is required.

— end note]

To form CI

^{''}

, each appearance of C's template parameters in the parameter mappings of the atomic constraints (including concept-dependent constraints) in CI

^{'}

is substituted with their respective arguments from the parameter mapping of CD and the arguments of CI.

CD is satisfied if CI

^{''}

is satisfied.

[Note 2:

Checking whether CI

^{''}

is satisfied can lead to further normalization of concept-dependent constraints.

— end note]

[Example 1: template<typename> concept C = true; template<typename T, template<typename> concept CC> concept D = CC<T>; template<typename U, template<typename> concept CT, template<typename, template<typename> concept> concept CU> int f() requires CU<U, CT>; int i = f<int, C, D>();

In this example, the associated constraints of f consist of a concept-dependent constraint whose expression is the concept-id CU<U, CT> with the mapping

U \mapsto U, CT \mapsto CT, CU \mapsto CU

The result of substituting D into this expression is D<U, CT>.

We consider the normal form of the resulting concept-id, which is CC<T> with the mapping

T \mapsto U, CC \mapsto CT

By recursion, C is substituted into CC<T>, and the result is normalized to the atomic constraint true, which is satisfied.

— end example]