[Bug libstdc++/109947] std::expected monadic operations do not support move-only error types yet

["redi at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109947

--- Comment #4 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Martin Seemann from comment #3)
> So it comes down to how to interpret the "Effects:" clause: Does "Equivalent
> to <statements involving std::move(value())>" mean  that all restrictions of
> `value()` apply transitively or is it merely an implementation hint?

The former.  The standard says:

Whenever the Effects element specifies that the semantics of some function F
are Equivalent to some code sequence, then the various elements are interpreted
as follows. If F’s semantics specifies any Constraints or Mandates elements,
then those requirements are logically imposed prior to the equivalent-to
semantics. Next, the semantics of the code sequence are determined by the
Constraints, Mandates, Preconditions, Effects, Synchronization, Postconditions,
Returns, Throws, Complexity, Remarks, and Error conditions specified for the
function invocations contained in the code sequence. The value returned from F
is specified by F’s Returns element, or if F has no Returns element, a non-void
return from F is specified by the return statements (8.7.4) in the code
sequence. If F’s semantics contains a Throws, Postconditions, or Complexity
element, then that supersedes any occurrences of that element in the code
sequence.


> (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.)

Yes. The spec were written by different people at different times.