[Bug libstdc++/109947] New: std::expected monadic operations do not support move-only error types yet
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
#include
int main()
{
using expected = std::expected>;
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.
[Bug libstdc++/109947] std::expected monadic operations do not support move-only error types yet
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109947 --- Comment #3 from Martin Seemann --- Thanks for pointing me to the LWG issue. It makes sense that the error type must be copyable for the `value()` overloads due to potentially throwing a `bad_expected_access` with the embedded error embedded. However, the monadic operations will never throw this exception. Consequently, the standard draft for the monadic operations (https://eel.is/c++draft/expected.object.monadic) does not contain any "Throws:" clause nor is copyability of the error type included in the "Constraints:" clause. So it comes down to how to interpret the "Effects:" clause: Does "Equivalent to " mean that all restrictions of `value()` apply transitively or is it merely an implementation hint? (Strangely enough, in the "Effects:" clause of `value_or()&&` the expression `std::move(**this)` is used instead of `std::move(value())`. Maybe this is an oversight/inconsistency of the standard.)
[Bug libstdc++/109947] std::expected monadic operations do not support move-only error types yet
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109947 Martin Seemann changed: What|Removed |Added Status|UNCONFIRMED |RESOLVED Resolution|--- |INVALID --- Comment #5 from Martin Seemann --- Thanks for the clarification! Now I am convinced that it is not a bug in libstdc++ (although I still doubt that the side-effects were intended when the committee formulated the "Effects" for monadic operations, but that's not relevant here). Marking as resolved and sorry for the noise.
[Bug libstdc++/109947] std::expected monadic operations do not support move-only error types yet
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109947 --- Comment #9 from Martin Seemann --- That's great news, looking forward to the next point release. Thank you very much for taking this to the committee and getting the process in motion so quickly!
