18 Concepts library [concepts]

18.5 Comparison concepts [concepts.compare]

18.5.2 Boolean testability [concept.booleantestable]

The exposition-only boolean-testable concept specifies the requirements on expressions that are convertible to bool and for which the logical operators ([expr.log.and], [expr.log.or], [expr.unary.op]) have the conventional semantics.
template<class T> concept boolean-testable-impl = convertible_­to<T, bool>; // exposition only
Let e be an expression such that decltype((e)) is T.
T models boolean-testable-impl only if:
  • either remove_­cvref_­t<T> is not a class type, or a search for the names operator&& and operator|| in the scope of remove_­cvref_­t<T> finds nothing; and
  • argument-dependent lookup ([basic.lookup.argdep]) for the names operator&& and operator|| with T as the only argument type finds no disqualifying declaration (defined below).
A disqualifying parameter is a function parameter whose declared type P
  • is not dependent on a template parameter, and there exists an implicit conversion sequence ([over.best.ics]) from e to P; or
  • is dependent on one or more template parameters, and either
    • P contains no template parameter that participates in template argument deduction ([temp.deduct.type]), or
    • template argument deduction using the rules for deducing template arguments in a function call ([temp.deduct.call]) and e as the argument succeeds.
A key parameter of a function template D is a function parameter of type cv X or reference thereto, where X names a specialization of a class template that has the same innermost enclosing non-inline namespace as D, and X contains at least one template parameter that participates in template argument deduction.
[Example 1:
In namespace Z { template<class> struct C {}; template<class T> void operator&&(C<T> x, T y); template<class T> void operator||(C<type_identity_t<T>> x, T y); } the declaration of Z​::​operator&& contains one key parameter, C<T> x, and the declaration of Z​::​operator|| contains no key parameters.
— end example]
A disqualifying declaration is
  • a (non-template) function declaration that contains at least one disqualifying parameter; or
  • a function template declaration that contains at least one disqualifying parameter, where
    • at least one disqualifying parameter is a key parameter; or
    • the declaration contains no key parameters; or
    • the declaration declares a function template to which no name is bound ([dcl.meaning]).
[Note 1:
The intention is to ensure that given two types T1 and T2 that each model boolean-testable-impl, the && and || operators within the expressions declval<T1>() && declval<T2>() and declval<T1>() || declval<T2>() resolve to the corresponding built-in operators.
— end note]
template<class T> concept boolean-testable = // exposition only boolean-testable-impl<T> && requires(T&& t) { { !std::forward<T>(t) } -> boolean-testable-impl; };
Let e be an expression such that decltype((e)) is T.
T models boolean-testable only if bool(e) == !bool(!e).
[Example 2:
The types bool, true_­type ([meta.type.synop]), int*, and bitset<N>​::​reference ([template.bitset]) model boolean-testable.
— end example]