The type `MoveOnlyForwardRange` violates the precondition stated in
`view.interface.general`. Specifically, the type passed to
`view_interface` shall model the `view` concept. In turn, this requires the
type to satisfy `movable` concept (and others), but this type
`MoveOnlyForwardRange` does not satisfy the `movable` concept.
Add a move assignment operator so that `MoveOnlyForwardRange` satisfies the
`movable` concept. While we're here, ensure the neighboring types that inherit
from `view_interface` also satisfy the `view` concept to avoid similar issues.
Fixes https://bugs.llvm.org/show_bug.cgi?id=50720
Reviewed By: Quuxplusone, Mordante, #libc
Differential Revision: https://reviews.llvm.org/D112631
c3cd5f5b4f