13 Templates [temp]

13.5 Template constraints [temp.constr]

13.5.2 Constraints [temp.constr.constr] Atomic constraints [temp.constr.atomic]

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 1: 
Atomic constraints are formed by constraint normalization.
— end 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 2: 
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 1: 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 } — end example]
As specified in [temp.over.link], if the validity or meaning of the program depends on whether two constructs are equivalent, and they are functionally equivalent but not equivalent, the program is ill-formed, no diagnostic required.
[Example 2: 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 } — end example]
— end note]
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 in the immediate context of the atomic constraint ([temp.deduct.general]), 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.
The constraint is satisfied if and only if evaluation of E results in true.
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 3: 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 — end example]