US276 24.03.1 p1.3 begin/end friends taking rvalues #272
Description
24.03.1
24.3.2
24.5.3
24.7.3.1
24.6.3.2
several
Several of the range views define non-template friend function begin/end overloads taking rvalues to satisfy the exposition-only forwarding-range concept. These have a couple of problems. First, the ones for subrange take subrange&&. That means that a const rvalue subrange fails to satisfy forwarding-range, which causes cbegin(subrange{...}) to be ill-formed.
The bigger problem is that since these functions are non-templates, whenever they get added to the overload set, the compiler will try conversions to these types (subrange, ref_view). The attempted conversions could lead to errors in theory.
Finally, class iota_view has iterator that can safely outlive the view that created them, so it too should be given begin/end friend functions that accept rvalues following the same pattern.
Proposed change:
In [range.access.begin]/p1.3, change the poison-pill overloads from:
template<class T> void begin(T&&) = delete; template/<class T> void begin(initializer_list<T>&&) = delete;
...to:
template<class T> void begin(T&&) = delete; template<class T> void begin(initializer_list<T>) = delete;
To the synopsis in [range.subrange]/p1, add:
template<class A, class B> concept same-ish = // exposition only same_as<A const, B const>;
In the class synopsis of subrange (same section), change the begin/end friend functions from:
friend constexpr I begin(subrange&& r) { return r.begin(); }
friend constexpr S end(subrange&& r) { return r.end(); }
...to:
friend constexpr I begin(same-ish<subrange> auto && r) { return r.begin(); }
friend constexpr S end(same-ish<subrange> auto && r) { return r.end(); }
In the synopsis of ref_view in [range.ref.view]/p1, change the begin/end friend functions from this:
friend constexpr iterator_t<R> begin(ref_view r)
{ return r.begin(); }
friend constexpr sentinel_t<R> end(ref_view r)
{ return r.end(); }
...to this (editors note: the use of same_as here instead of same-ish is intentional; likewise for the use of pass-by-value):
friend constexpr iterator_t<R> begin(same_as<ref_view> auto r)
{ return r.begin(); }
friend constexpr sentinel_t<R> end(same_as<ref_view> auto r)
{ return r.end(); }
To the class synopsis of iota_view in [range.iota.view], add the following begin/end friend functions:
friend constexpr W begin(same-ish<iota_view> auto && r) { return r.begin(); }
friend constexpr auto end(same-ish<iota_view> auto && r) { return r.end(); }
See ericniebler/stl2#592.