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From: "aemseemann at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug libstdc++/109947] New: std::expected monadic operations do not
 support move-only error types yet
Date: Tue, 23 May 2023 19:12:45 +0000
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X-Bugzilla-Product: gcc
X-Bugzilla-Component: libstdc++
X-Bugzilla-Version: 13.1.0
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D109947

            Bug ID: 109947
           Summary: std::expected monadic operations do not support
                    move-only error types yet
           Product: gcc
           Version: 13.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: aemseemann at gmail dot com
  Target Milestone: ---

GCC13 introduce monadic operations for `std::expected`, including r-value
ref-qualified overloads, which suggests that it should be possible to use an
expected with a move-only value or error type.

However, the following [example](https://godbolt.org/z/aoWeaqoGz) does not
compile due to an attempt to use unique_ptr's the deleted copy constructor:

```cpp
#include <expected>
#include <memory>

int main()=20
{=20=20=20
    using expected =3D std::expected<int, std::unique_ptr<int>>;

    expected e{42};
    std::move(e).and_then([](auto&&) -> expected {
        return 0;
    });

    return 0;
}

```

The issue seems to be the use of `std::move(value())` in the &&-qualified
overloads of the monadic operations (e.g.
[here](https://github.com/gcc-mirror/gcc/blob/master/libstdc%2B%2B-v3/inclu=
de/std/expected#L880)
which selects the `value() &` overload that in turn attempts a copy of the
error type in the  [exception
path](https://github.com/gcc-mirror/gcc/blob/master/libstdc%2B%2B-v3/includ=
e/std/expected#L740).

When replacing the value access with `std::move(*this).value()` the example
compiles successfully.=