The *normal form* of an expression E is
a constraint that is defined as follows:

- The normal form of an expression ( E ) is the normal form of E.
- The normal form of an expression E1 || E2 is the disjunction of the normal forms of E1 and E2.
- The normal form of an expression E1 && E2 is the conjunction of the normal forms of E1 and E2.
- The normal form of a concept-id C<A, A, ..., A>
is the normal form of the constraint-expression of C,
after substituting A, A, ..., A for
C's respective template parameters in the
parameter mappings in each atomic constraint. If any such substitution results in an invalid type or expression,
the program is ill-formed; no diagnostic is required. [ Example:]
template<typename T> concept A = T::value || true; template<typename U> concept B = A<U*>; template<typename V> concept C = B<V&>;

Normalization of B's constraint-expression is valid and results in T::value (with the mapping ) ∨ true (with an empty mapping), despite the expression T::value being ill-formed for a pointer type T. Normalization of C's constraint-expression results in the program being ill-formed, because it would form the invalid type V&* in the parameter mapping. —*end example* - The normal form of any other expression E is the atomic constraint whose expression is E and whose parameter mapping is the identity mapping.

[ Note

: *end note*

]Normalization of constraint-expressions
is performed
when determining the associated constraints ([temp.constr.constr])
of a declaration
and
when evaluating the value of an id-expression
that names a concept specialization ([expr.prim.id]).

— [ Example

:

—*end example*

]template<typename T> concept C1 = sizeof(T) == 1; template<typename T> concept C2 = C1<T> && 1 == 2; template<typename T> concept C3 = requires { typename T::type; }; template<typename T> concept C4 = requires (T x) { ++x; } template<C2 U> void f1(U); // #1 template<C3 U> void f2(U); // #2 template<C4 U> void f3(U); // #3

—