https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109947
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()
{
using expected = 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/include/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/include/std/expected#L740).
When replacing the value access with `std::move(*this).value()` the example
compiles successfully.
