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Section: 26.3.2 [range.access.begin], 26.3.3 [range.access.end], 26.3.6 [range.access.rbegin], 26.3.7 [range.access.rend] Status: Resolved Submitter: Casey Carter Opened: 2019-08-15 Last modified: 2021-06-23
Priority: 3
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Discussion:
The specification of ranges::begin in 26.3.2 [range.access.begin] includes a "poison pill" overload:
which exists to create an ambiguity with the non-member initializer_list overload of begin in namespace std (17.10.2 [initializer.list.syn]) when performing unqualified lookup, since specializations of initializer_list should not satisfy forwarding-range (26.4.2 [range.range]). The design intent is that const specializations of initializer_list should also not satisfy forwarding-range, although they are rare enough beasts that they were overlooked when this wording is written.template<class T> void begin(initializer_list<T>&&) = delete;
ranges::end (26.3.3 [range.access.end]) has a similar poison pill for initializer_list, which should be changed consistently.
Notably ranges::rbegin (26.3.6 [range.access.rbegin]) and ranges::rend (26.3.6 [range.access.rbegin]) as currently specified accept rvalue initializer_list arguments; they find the initializer_list overloads of std::rbegin and std::rend (25.7 [iterator.range]) via ADL. While I can't put my finger on anything in particular that's broken by this behavior, it seems wise to make rbegin and rend consistent with begin and end for initializer_list until and unless we discover a reason to do otherwise.
[2019-10 Priority set to 3 after reflector discussion]
[2021-06-23 Resolved by adoption of P2091R0 in Prague. Status changed: New → Resolved.]
Proposed resolution:
This wording is relative to N4830.
Modify 26.3.2 [range.access.begin] as follows:
-1- The name ranges::begin denotes a customization point object (16.3.3.3.5 [customization.point.object]). The expression ranges::begin(E) for some subexpression E is expression-equivalent to:
(1.1) — E + 0 if E is an lvalue of array type (6.8.4 [basic.compound]).
(1.2) — Otherwise, if E is an lvalue, decay-copy(E.begin()) if it is a valid expression and its type I models input_or_output_iterator.
(1.3) — Otherwise, decay-copy(begin(E)) if it is a valid expression and its type I models input_or_output_iterator with overload resolution performed in a context that includes the declarations:
and does not include a declaration of ranges::begin.template<class T> void begin(T&&) = delete; template<class T> void begin(initializer_list<T>&&) = delete;[…]
Modify 26.3.3 [range.access.end] as follows:
-1- The name ranges::end denotes a customization point object (16.3.3.3.5 [customization.point.object]). The expression ranges::end(E) for some subexpression E is expression-equivalent to:
(1.1) — E + extent_v<T> if E is an lvalue of array type (6.8.4 [basic.compound]) T.
(1.2) — Otherwise, if E is an lvalue, decay-copy(E.end()) if it is a valid expression and its type S models sentinel_for<decltype(ranges::begin(E))>
(1.3) — Otherwise, decay-copy(end(E)) if it is a valid expression and its type S models sentinel_for<decltype(ranges::begin(E))> with overload resolution performed in a context that includes the declarations:
and does not include a declaration of ranges::end.template<class T> void end(T&&) = delete; template<class T> void end(initializer_list<T>&&) = delete;[…]
Modify 26.3.6 [range.access.rbegin] as follows:
-1- The name ranges::rbegin denotes a customization point object (16.3.3.3.5 [customization.point.object]). The expression ranges::rbegin(E) for some subexpression E is expression-equivalent to:
(1.1) — If E is an lvalue, decay-copy(E.rbegin()) if it is a valid expression and its type I models input_or_output_iterator.
(1.2) — Otherwise, decay-copy(rbegin(E)) if it is a valid expression and its type I models input_or_output_iterator with overload resolution performed in a context that includes the declarations:
and does not include a declaration of ranges::rbegin.template<class T> void rbegin(T&&) = delete; template<class T> void rbegin(initializer_list<T>) = delete;[…]
Modify 26.3.7 [range.access.rend] as follows:
-1- The name ranges::rend denotes a customization point object (16.3.3.3.5 [customization.point.object]). The expression ranges::rend(E) for some subexpression E is expression-equivalent to:
(1.1) — If E is an lvalue, decay-copy(E.rend()) if it is a valid expression and its type S models
sentinel_for<decltype(ranges::rbegin(E))>(1.2) — Otherwise, decay-copy(rend(E)) if it is a valid expression and its type S models
with overload resolution performed in a context that includes the declarations:sentinel_for<decltype(ranges::rbegin(E))>and does not include a declaration of ranges::rend.template<class T> void rend(T&&) = delete; template<class T> void rend(initializer_list<T>) = delete;[…]