[ 2024-07-25 Status changed: New → Resolved. ]

Resolved by p2968r2, approved in St. Louis.

[ 2023-11-03; Reflector poll ]

Set priority to 4 after reflector poll. "Specify it as code." "P2968 should fix this."

[tuple.creation]/5 currently says:

[…] When an argument in t is ignore, assigning any value to the corresponding tuple element has no effect.

which is unimplementable for volatile-qualified bit-field glvalues.

In order to passing such a value to the operator= function, a distinct object needs to be created and thus the read from the volatile glvalue, which is a side effect ([intro.execution]/7), is unavoidable.

P2968R0 addresses the impossibility of assignment from void values, but doesn't talk about volatile bit-fields. Perhaps we should explicitly say that the program is ill-formed if a volatile bit-field value is assigned to std::ignore (which is implemented in libstdc++ and MSVC STL, but not in libc++).