[Bug libstdc++/105720] New: std::views::split_view wrong behaviour in case of partial match

[Bug libstdc++/105720] New: std::views::split_view wrong behaviour in case of partial match

[andij.cr at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105720

            Bug ID: 105720
           Summary: std::views::split_view wrong behaviour in case of
                    partial match
           Product: gcc
           Version: 10.3.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: andij.cr at gmail dot com
  Target Milestone: ---

compiled with 
g++-10 -std=c++20 split_view_wrong.cpp -lfmt

godbolt link https://gcc.godbolt.org/z/47TxWovd4

fmtlib used for exposition only

    #include <fmt/printf.h>
    #include <fmt/ranges.h>
    #include <ranges>
    #include <string_view>

    auto words_no_bug = std::string_view{"Hello-_-C++-_-20-_-!-_-"};
    auto words_bug = std::string_view{"Hello--_-C++-_-20-_-!-_-"};
    auto delim = std::string_view{"-_-"};

    // needed because split_view is lazy in gcc 10.3
    auto range_to_str = [](auto &&r) {
      return fmt::format("{}", fmt::join(r, ""));
    };

    int main() {
      fmt::print("no bug: '{}' tokens: {}\n", words_no_bug,
                     words_no_bug | std::views::split(delim) |
                     std::views::transform(range_to_str));

      fmt::print("bug: '{}' tokens: {}\n", words_bug,
                     words_bug | std::views::split(delim) |
                     std::views::transform(range_to_str));
    }

this code applies split to tokenize a text

compiled with gcc-10.3 it wrongly produces

    no bug: 'Hello-_-C++-_-20-_-!-_-' tokens: ["Hello", "C++", "20", "!"]
    bug: 'Hello--_-C++-_-20-_-!-_-' tokens: ["Hello-", "20", "!"]

while compiled with gcc-11.3 is correctly produces 

    no bug: 'Hello-_-C++-_-20-_-!-_-' tokens: ["Hello", "C++", "20", "!"]
    bug: 'Hello--_-C++-_-20-_-!-_-' tokens: ["Hello-", "C++", "20", "!"]


notice how the substring "--_-C++" instead of being split in ["-", "C++"]
is split as ["-"], skipping the "C++" token.

it's fixed from gcc-11, but i couldn't find a mention in the release notes
about it

[Bug libstdc++/105720] std::views::split_view wrong behaviour in case of partial match

[ppalka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105720

Patrick Palka <ppalka at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|---                         |FIXED
             Status|UNCONFIRMED                 |RESOLVED
                 CC|                            |ppalka at gcc dot gnu.org
   Target Milestone|---                         |10.4

--- Comment #1 from Patrick Palka <ppalka at gcc dot gnu.org> ---
I think you're running into the LWG issue https://wg21.link/lwg3505 which has
been adopted as a defect report against C++20, and implemented for libstdc++
10.4 (not yet released) and 11+ (as you've noticed).  It's been backported to
the 10 release branch as r10-9740-g4c17b2bbbc1bef on April 20, 2021, so any
snapshot of libstdc++ 10 after that date should behave correctly.