[iterator.concept.writable]

template<class Out, class T> concept indirectly_writable = requires(Out&& o, T&& t) { *o = std::forward<T>(t); / not required to be equality-preserving *std::forward<Out>(o) = std::forward<T>(t); / not required to be equality-preserving const_cast<const iter_reference_t<Out>&>(*o) = std::forward<T>(t); / not required to be equality-preserving const_cast<const iter_reference_t<Out>&>(*std::forward<Out>(o)) = std::forward<T>(t); / not required to be equality-preserving };

Let E be an expression such that decltype(E)) is T, and let o be a dereferenceable object of type Out.

Out and T model indirectly_writable<Out, T> only if:

(2.1)
If Out and T model indirectly_readable<Out> && same_as<iter_value_t<Out>, decay_t<T>>, then *o after any above assignment is equal to the value of E before the assignment.

If E is an xvalue ([basic.lval]), the resulting state of the object it denotes is valid but unspecified ([lib.types.movedfrom]).

[Note 2:

indirectly_writable has the awkward const_cast expressions to reject iterators with prvalue non-proxy reference types that permit rvalue assignment but do not also permit const rvalue assignment.

Consequently, an iterator type I that returns std::string by value does not model indirectly_writable<I, std::string>.

— end note]