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Section: 26.7.15.2 [range.join.with.view] Status: Open Submitter: Hewill Kang Opened: 2023-08-21 Last modified: 2024-03-19
Priority: 3
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Discussion:
The issue that concat_view implicitly breaks equality-preserving by concatenating range of references with range
of prvalues seems to be reflected in join_view as well.
string&& and the reference of the pattern range is prvalue
string, dereferencing its iterator will move the elements of the inner range to the returned string,
which makes the second dereference get an empty string (demo):
vector v1{"hello"s};
vector v2{"world"s};
vector v{v1 | views::as_rvalue, v2 | views::as_rvalue};
auto pattern = views::iota(0, 1) | views::transform([](int) { return ", "s; });
ranges::forward_range auto joined = v | views::join_with(pattern);
fmt::print("{}\n", joined); // ["hello", ", ", "world"]
fmt::print("{}\n", joined); // ["", ", ", ""]
Not sure if we should ban such less common case.
[2023-10-30; Reflector poll]
Set priority to 3 after reflector poll. Send to SG9.
[2024-03-19; Tokyo: feedback from SG9]
SG9 believe that LWG3971 is a use-after-move and is working as intended.
We further believe that matching the behavior of concat_view
(which is the current behavior) is important. This is not a defect.
Proposed resolution: