[ Note

: *end note*

]This subclause defines the meaning of constraints on template arguments.

The abstract syntax and satisfaction rules are defined
in [temp.constr.constr].

Constraints are associated with declarations in [temp.constr.decl].

Declarations are partially ordered by their associated constraints ([temp.constr.order]).

— A *constraint* is a sequence of logical operations and
operands that specifies requirements on template arguments.

The operands of a logical operation are constraints.

There are three different kinds of constraints:

In order for a constrained template to be instantiated ([temp.spec]),
its associated constraints
shall be satisfied as described in the following subclauses.

[ Note

: *end note*

]Forming the name of a specialization of
a class template,
a variable template, or
an alias template ([temp.names])
requires the satisfaction of its constraints.

Overload resolution
requires the satisfaction of constraints
on functions and function templates.

— There are two binary logical operations on constraints: conjunction
and disjunction.

A *conjunction* is a constraint taking two
operands.

If that is not satisfied, the conjunction is not satisfied.

Otherwise, the conjunction is satisfied if and only if the second
operand is satisfied.

A *disjunction* is a constraint taking two
operands.

If that is satisfied, the disjunction is satisfied.

Otherwise, the disjunction is satisfied if and only if the second
operand is satisfied.

[ Example

: *end example*

]template<typename T> constexpr bool get_value() { return T::value; } template<typename T> requires (sizeof(T) > 1) && (get_value<T>()) void f(T); // has associated constraint sizeof(T) > 1 ∧ get_value<T>() void f(int); f('a'); // OK: calls f(int)

In the satisfaction of the associated constraints
of f, the constraint sizeof(char) > 1 is not satisfied;
the second operand is not checked for satisfaction.

— [ Note

: *end note*

]A logical negation expression ([expr.unary.op]) is an atomic constraint;
the negation operator is not treated as a logical operation on constraints.

As a result, distinct negation constraint-expressions
that are equivalent under [temp.over.link]
do not subsume one another under [temp.constr.order].

Furthermore, if substitution to determine
whether an atomic constraint is satisfied ([temp.constr.atomic])
encounters a substitution failure, the constraint is not satisfied,
regardless of the presence of a negation operator.

[ Example

— : *end example*

]template <class T> concept sad = false; template <class T> int f1(T) requires (!sad<T>); template <class T> int f1(T) requires (!sad<T>) && true; int i1 = f1(42); // ambiguous, !sad<T> atomic constraint expressions ([temp.constr.atomic]) // are not formed from the same expression template <class T> concept not_sad = !sad<T>; template <class T> int f2(T) requires not_sad<T>; template <class T> int f2(T) requires not_sad<T> && true; int i2 = f2(42); // OK, !sad<T> atomic constraint expressions both come from not_sad template <class T> int f3(T) requires (!sad<typename T::type>); int i3 = f3(42); // error: associated constraints not satisfied due to substitution failure template <class T> concept sad_nested_type = sad<typename T::type>; template <class T> int f4(T) requires (!sad_nested_type<T>); int i4 = f4(42); // OK, substitution failure contained within sad_nested_type

Here,
requires (!sad<typename T::type>) requires
that there is a nested type that is not sad,
whereas
requires (!sad_nested_type<T>) requires
that there is no sad nested type.

— An *atomic constraint* is formed from
an expression E
and a mapping from the template parameters
that appear within E to
template arguments that are formed via substitution during constraint normalization
in the declaration of a constrained entity (and, therefore, can involve the
unsubstituted template parameters of the constrained entity),
called the *parameter mapping* ([temp.constr.decl]).

[ Note ]

Two atomic constraints, and , are
*identical*
if they are formed from the same appearance of the same
expression
and if, given a hypothetical template A
whose template-parameter-list consists of
template-parameters corresponding and equivalent ([temp.over.link]) to
those mapped by the parameter mappings of the expression,
a template-id naming A
whose template-arguments are
the targets of the parameter mapping of
is the same ([temp.type]) as
a template-id naming A
whose template-arguments are
the targets of the parameter mapping of .

[ Note

: *end note*

]The comparison of parameter mappings of atomic constraints
operates in a manner similar to that of declaration matching
with alias template substitution ([temp.alias]).

[ Example

: *end example*

]template <unsigned N> constexpr bool Atomic = true; template <unsigned N> concept C = Atomic<N>; template <unsigned N> concept Add1 = C<N + 1>; template <unsigned N> concept AddOne = C<N + 1>; template <unsigned M> void f() requires Add1<2 * M>; template <unsigned M> int f() requires AddOne<2 * M> && true; int x = f<0>(); // OK, the atomic constraints from concept C in both fs are Atomic<N> // with mapping similar to template <unsigned N> struct WrapN; template <unsigned N> using Add1Ty = WrapN<N + 1>; template <unsigned N> using AddOneTy = WrapN<N + 1>; template <unsigned M> void g(Add1Ty<2 * M> *); template <unsigned M> void g(AddOneTy<2 * M> *); void h() { g<0>(nullptr); // OK, there is only one g }—

This similarity includes the situation where a program is ill-formed, no diagnostic required,
when the meaning of the program depends on whether two constructs are equivalent,
and they are functionally equivalent but not equivalent.

[ Example

— : *end example*

]template <unsigned N> void f2() requires Add1<2 * N>; template <unsigned N> int f2() requires Add1<N * 2> && true; void h2() { f2<0>(); // ill-formed, no diagnostic required: // requires determination of subsumption between atomic constraints that are // functionally equivalent but not equivalent }—

To determine if an atomic constraint is
*satisfied*,
the parameter mapping and template arguments are
first substituted into its expression.

If substitution results in an invalid type or expression,
the constraint is not satisfied.

Otherwise, the lvalue-to-rvalue conversion
is performed if necessary,
and E shall be a constant expression of type bool.

If, at different points in the program, the satisfaction result is different
for identical atomic constraints and template arguments,
the program is ill-formed, no diagnostic required.

[ Example

: *end example*

]template<typename T> concept C = sizeof(T) == 4 && !true; // requires atomic constraints sizeof(T) == 4 and !true template<typename T> struct S { constexpr operator bool() const { return true; } }; template<typename T> requires (S<T>{}) void f(T); // #1 void f(int); // #2 void g() { f(0); // error: expression S<int>{} does not have type bool } // while checking satisfaction of deduced arguments of #1; // call is ill-formed even though #2 is a better match—

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:

- Otherwise, if there is a single introduced constraint-expression, the associated constraints are the normal form of that expression.
- Otherwise, the associated constraints are the normal form of a logical AND expression whose operands are in the following order:
- the constraint-expression introduced by each type-constraint in the declaration's template-parameter-list, in order of appearance, and
- the constraint-expression introduced by a requires-clause following a template-parameter-list, and
- the constraint-expression introduced by each type-constraint in the parameter-type-list of a function declaration, and
- 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

: *end example*

]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>;

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);

template<C1 T> requires C2<T> void f6(); template<C2 T> requires C1<T> void f7();—

When determining whether a given introduced
constraint-expression of a declaration
in an instantiated specialization of a templated class
is equivalent ([temp.over.link]) to the corresponding
constraint-expression of a declaration
outside the class body,
is instantiated.

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

[ Note

: *end note*

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

— [ Example

: *end example*

]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.

— 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

—

A constraint P *subsumes* a constraint Q
if and only if,
for every disjunctive clause
in the disjunctive normal form130
of P, subsumes every conjunctive clause
in the conjunctive normal form131
of Q, where

- a disjunctive clause subsumes a conjunctive clause if and only if there exists an atomic constraint in for which there exists an atomic constraint in such that subsumes , and
- an atomic constraint A subsumes another atomic constraint B if and only if A and B are identical using the rules described in [temp.constr.atomic].

[ Note

: *end note*

]The subsumption relation defines a partial ordering on constraints.

This partial ordering is used to determine

— - the best viable candidate of non-template functions ([over.match.best]),
- the address of a non-template function ([over.over]),
- the matching of template template arguments,
- the partial ordering of class template specializations, and
- the partial ordering of function templates.

A declaration D1 is
*at least as constrained* as
a declaration D2 if

- D1 and D2 are both constrained declarations and D1's associated constraints subsume those of D2; or
- D2 has no associated constraints.

A declaration D1 is *more constrained*
than another declaration D2 when D1 is at least as
constrained as D2, and D2 is not at least as
constrained as D1.

[ Example

: *end example*

]template<typename T> concept C1 = requires(T t) { --t; }; template<typename T> concept C2 = C1<T> && requires(T t) { *t; }; template<C1 T> void f(T); // #1 template<C2 T> void f(T); // #2 template<typename T> void g(T); // #3 template<C1 T> void g(T); // #4 f(0); // selects #1 f((int*)0); // selects #2 g(true); // selects #3 because C1<bool> is not satisfied g(0); // selects #4—