[temp.constr.concept]

# 13 Templates [[temp]](./#temp)

## 13.5 Template constraints [[temp.constr]](temp.constr#concept)

### 13.5.2 Constraints [[temp.constr.constr]](temp.constr.constr#temp.constr.concept)

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

[1](#1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/templates.tex#L1857)

A [*concept-dependent constraint*](#def:constraint,concept-dependent "13.5.2.4 Concept-dependent constraints [temp.constr.concept]") CD is
an atomic constraint whose expression is a concept-id CI whose[*concept-name*](temp.concept#nt:concept-name "13.7.9 Concept definitions [temp.concept]") names a dependent concept named C[.](#1.sentence-1)

[2](#2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/templates.tex#L1862)

To determine if CD is[*satisfied*](#def:constraint,satisfaction,concept-dependent "13.5.2.4 Concept-dependent constraints [temp.constr.concept]"),
the parameter mapping and template arguments are first
substituted into C[.](#2.sentence-1)

If substitution results in an invalid concept-id in
the immediate context of the constraint ([[temp.deduct.general]](temp.deduct.general "13.10.3.1 General")),
the constraint is not satisfied[.](#2.sentence-2)

Otherwise, let CI′ be
the normal form ([[temp.constr.normal]](temp.constr.normal "13.5.4 Constraint normalization")) of the concept-id
after substitution of C[.](#2.sentence-3)

[*Note [1](#note-1)*:

Normalization of CI might be ill-formed; no diagnostics is required[.](#2.sentence-4)

— *end note*]

[3](#3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/templates.tex#L1877)

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[.](#3.sentence-1)

[4](#4)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/templates.tex#L1886)

CD is satisfied if CI′′ is satisfied[.](#4.sentence-1)

[*Note [2](#note-2)*:

Checking whether CI′′ is satisfied
can lead to further normalization of concept-dependent constraints[.](#4.sentence-2)

— *end note*]

[*Example [1](#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 mappingU↦U,CT↦CT,CU↦CU[.](#4.sentence-3)

The result of substituting D into this expression is D<U, CT>[.](#4.sentence-4)

We consider the normal form of the resulting concept-id,
which is CC<T> with the mappingT↦U,CC↦CT[.](#4.sentence-5)

By recursion, C is substituted into CC<T>, and the result
is normalized to the atomic constraint true, which is satisfied[.](#4.sentence-6)

— *end example*]