This reverts commit 6a7bd45a8e.
PR #17495 was originally merged in an incomplete state. Since there are
non-trivial obstacles to completing it, revert the incomplete changes
instead.
See PR #20670 for more information.
Introduced in PR #9331, this was changed from a normal bug fix to a
-preview switch because enabling it triggered a suprious "statement is
not reachable" warning in Phobos. That warning has since been removed
(in PR #15568).
