[range.nonprop.cache]

Some types in [range.adaptors] are specified in terms of an exposition-only class template non-propagating-cache.

non-propagating-cache<T> behaves exactly like optional<T> with the following differences:

(1.1)
non-propagating-cache<T> constrains its type parameter T with is_object_v<T>.
(1.2)
The copy constructor is equivalent to: constexpr non-propagating-cache(const non-propagating-cache&) noexcept {}
(1.3)
The move constructor is equivalent to: constexpr non-propagating-cache(non-propagating-cache&& other) noexcept { other.reset(); }
(1.4)
The copy assignment operator is equivalent to: constexpr non-propagating-cache& operator=(const non-propagating-cache& other) noexcept { if (addressof(other) != this) reset(); return *this; }
(1.5)
The move assignment operator is equivalent to: constexpr non-propagating-cache& operator=(non-propagating-cache&& other) noexcept { reset(); other.reset(); return *this; }
(1.6)
non-propagating-cache<T> has an additional member function template specified as follows:
🔗
template<class I> constexpr T& emplace-deref(const I& i); / exposition only

Mandates: The declaration T t(*i); is well-formed for some invented variable t.

[Note 1:
If *i is a prvalue of type cv T, there is no requirement that it is movable ([dcl.init.general]).
— end note]
Effects: Calls reset().

Then direct-non-list-initializes the contained value with *i.
Postconditions: *this contains a value.
Returns: A reference to the new contained value.
Throws: Any exception thrown by the initialization of the contained value.
Remarks: If an exception is thrown during the initialization of T, *this does not contain a value, and the previous value (if any) has been destroyed.

[Note 2:

non-propagating-cache enables an input view to temporarily cache values as it is iterated over.

— end note]