std::indirectly_readable

template< class In >
  concept __IndirectlyReadableImpl = // exposition only
    requires(const In in) {
      typename std::iter_value_t<In>;
      typename std::iter_reference_t<In>;
      typename std::iter_rvalue_reference_t<In>;
      { *in } -> std::same_as<std::iter_reference_t<In>>;
      { ranges::iter_move(in) } -> std::same_as<std::iter_rvalue_reference_t<In>>;
    } &&
    std::common_reference_with<
      std::iter_reference_t<In>&&, std::iter_value_t<In>&
    > &&
    std::common_reference_with<
      std::iter_reference_t<In>&&, std::iter_rvalue_reference_t<In>&&
    > &&
    std::common_reference_with<
      std::iter_rvalue_reference_t<In>&&, const std::iter_value_t<In>&
    >;

template< class In >
  concept indirectly_readable =
    __IndirectlyReadableImpl<std::remove_cvref_t<In>>;

The concept indirectly_readable is modeled by types that are readable by applying operator*, such as pointers, smart pointers, and input iterators.

Semantic requirements

Given a value i of type I, I models indirectly_readable only if all concepts it subsumes are modeled and the expression *i is equality preserving.

Equality preservation

An expression is equality preserving if it results in equal outputs given equal inputs.

• The inputs to an expression consist of its operands.
• The outputs of an expression consist of its result and all operands modified by the expression (if any).

In specification of standard concepts, operands are defined as the largest subexpressions that include only:

• an id-expression, and
• invocations of std::move, std::forward, and std::declval.

The cv-qualification and value category of each operand is determined by assuming that each template type parameter denotes a cv-unqualified complete non-array object type.

Every expression required to be equality preserving is further required to be stable: two evaluations of such an expression with the same input objects must have equal outputs absent any explicit intervening modification of those input objects.

Unless noted otherwise, every expression used in a requires-expression is required to be equality preserving and stable, and the evaluation of the expression may modify only its non-constant operands. Operands that are constant must not be modified.