From Jason Turner

[range.access.rbegin]

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+ ### `ranges::rbegin` <a id="range.access.rbegin">[[range.access.rbegin]]</a>
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+
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+ The name `ranges::rbegin` denotes a customization point object
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+ [[customization.point.object]].
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+
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+ Given a subexpression `E` with type `T`, let `t` be an lvalue that
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+ denotes the reified object for `E`. Then:
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+
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+ - If `E` is an rvalue and `enable_borrowed_range<remove_cv_t<T>>` is
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+ `false`, `ranges::rbegin(E)` is ill-formed.
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+ - Otherwise, if `T` is an array type [[basic.compound]] and
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+ `remove_all_extents_t<T>` is an incomplete type, `ranges::rbegin(E)`
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+ is ill-formed with no diagnostic required.
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+ - Otherwise, if `decay-copy(t.rbegin())` is a valid expression whose
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+ type models `input_or_output_iterator`, `ranges::rbegin(E)` is
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+ expression-equivalent to `decay-copy(t.rbegin())`.
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+ - Otherwise, if `T` is a class or enumeration type and
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+ `decay-copy(rbegin(t))` is a valid expression whose type models
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+ `input_or_output_iterator` with overload resolution performed in a
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+ context in which unqualified lookup for `rbegin` finds only the
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+ declarations
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+ ``` cpp
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+ void rbegin(auto&) = delete;
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+ void rbegin(const auto&) = delete;
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+ ```
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+
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+ then `ranges::rbegin(E)` is expression-equivalent to
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+ `decay-copy(rbegin(t))` with overload resolution performed in the
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+ above context.
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+ - Otherwise, if both `ranges::begin(t)` and `ranges::end(t)` are valid
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+ expressions of the same type which models `bidirectional_iterator`
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+ [[iterator.concept.bidir]], `ranges::rbegin(E)` is
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+ expression-equivalent to `make_reverse_iterator(ranges::end(t))`.
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+ - Otherwise, `ranges::rbegin(E)` is ill-formed.
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+
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+ [*Note 1*: Diagnosable ill-formed cases above result in substitution
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+ failure when `ranges::rbegin(E)` appears in the immediate context of a
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+ template instantiation. — *end note*]
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+
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+ [*Note 2*: Whenever `ranges::rbegin(E)` is a valid expression, its type
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+ models `input_or_output_iterator`. — *end note*]
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+