18 Concepts library [concepts]

18.4 Language-related concepts [concepts.lang]

18.4.4 Concept convertible_­to [concept.convertible]

1
#
Given types From and To and an expression E such that decltype((E)) is add_­rvalue_­reference_­t<From>, convertible_­to<From, To> requires E to be both implicitly and explicitly convertible to type To.
The implicit and explicit conversions are required to produce equal results.
🔗
template<class From, class To> concept convertible_­to = is_convertible_v<From, To> && requires(add_rvalue_reference_t<From> (&f)()) { static_cast<To>(f()); };
2
#
Let FromR be add_­rvalue_­reference_­t<From> and test be the invented function: 
To test(FromR (&f)()) {
  return f();
}
and let f be a function with no arguments and return type FromR such that f() is equality-preserving.
Types From and To model convertible_­to<From, To> only if:
  • (2.1)
    To is not an object or reference-to-object type, or static_­cast<To>(f()) is equal to test(f).
  • (2.2)
    FromR is not a reference-to-object type, or
    • (2.2.1)
      If FromR is an rvalue reference to a non const-qualified type, the resulting state of the object referenced by f() after either above expression is valid but unspecified ([lib.types.movedfrom]).
    • (2.2.2)
      Otherwise, the object referred to by f() is not modified by either above expression.