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3864.zip over range of reference to an abstract type
Section: 25.7.25 [range.zip] Status: New Submitter: Barry Revzin Opened: 2023-01-28 Last modified: 2023-02-06
Priority: 4
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Discussion:
Consider:
#include <ranges>
struct Abstract {
virtual ~Abstract() = default;
virtual int f() = 0;
};
struct Concrete : Abstract {
int f() override { return 42; }
};
int main() {
Concrete c[10];
auto xformed = c | std::views::transform([](Concrete& c) -> Abstract& {
return c;
});
for (Abstract& a : xformed) { } // ok
auto zipped = std::views::zip(xformed);
for (auto&& [a] : zipped) { } // error
}
Here, xformed is a range whose reference type is Abstract& and whose value_type is Abstract. Even though you can't actually create a value of that value_type, that's okay here, because no code is actually trying to do so.
zipped is a range whose reference type is std::tuple<Abstract&> and whose value_type is std::tuple<Abstract>. No code here is actually trying to construct a value_type either, but this code fails because simply instantiating std::tuple<Abstract> is an error. There's no other possible value_type for zipped to have, std::tuple<Abstract> is correct — it's just that it happens to be an ill-formed type in this context. There are workarounds for this case — you would have to make xformed be a range of Abstract* or, probably better, a range of reference_wrapper<Abstract> instead. This is unfortunate because many (most?) algorithms don't actually make any use of a range's value_type. The ones that do (like ranges::min) obviously could not work, but currently we end up rejecting all uses. Probably the only possible way to make this work is to allow value_type to be void (or absent), but it is currently a fairly fundamental type due to its use in indirectly_readable to identify input iterators.
[2023-02-06; Reflector poll]
Set priority to 4 after reflector poll. Several votes for NAD. Maybe tuple<Abstract> should be explicitly made ill-formed (currently seems underspecified) or should be "disabled" like invalid hash specializations and formatters.
Proposed resolution:
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