7 Expressions [expr]

7.5 Primary expressions [expr.prim]

7.5.7 Requires expressions [expr.prim.req]

7.5.7.4 Nested requirements [expr.prim.req.nested]

nested-requirement:
requires constraint-expression ;
1
#
A nested-requirement can be used to specify additional constraints in terms of local parameters.
The constraint-expression shall be satisfied ([temp.constr.decl]) by the substituted template arguments, if any.
Substitution of template arguments into a nested-requirement does not result in substitution into the constraint-expression other than as specified in [temp.constr.constr].
[Example
: 
template<typename U> concept C = sizeof(U) == 1;

template<typename T> concept D = requires (T t) {
  requires C<decltype (+t)>;
};
D<T> is satisfied if sizeof(decltype (+t)) == 1 ([temp.constr.atomic]).
— end example
]
2
#
A local parameter shall only appear as an unevaluated operand within the constraint-expression.
[Example
: 
template<typename T> concept C = requires (T a) {
  requires sizeof(a) == 4;      // OK
  requires a == 0;              // error: evaluation of a constraint variable
};
— end example
]