18 Concepts library [concepts]

18.4 Language-related concepts [concepts.lang]

18.4.4 Concept convertible_to [concept.convertible]

Given types From and To and an expression E whose type and value category are the same as those of declval<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 { static_cast<To>(declval<From>()); };
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:
  • To is not an object or reference-to-object type, or static_cast<To>(f()) is equal to test(f).
  • FromR is not a reference-to-object type, or
    • 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]).
    • Otherwise, the object referred to by f() is not modified by either above expression.