The current implementation of `forward_list::resize` does not meet the
strong exception safety guarantee required by [forward.list.modifiers]:
If an exception is thrown by any of these member functions there is no
effect on the container.
This patch refactors `resize()` to provide strong exception safety and
introduces additional tests to validate the strong exception guarantees
for other `forward_list` modifiers.
Fixes#118366.
We've been improving these the tests for vector quite a bit and we are
probably not done improving our container tests. Formatting everything
at once will make subsequent reviews easier.
When inserting nodes into a forward list, each new node is allocated but
not constructed. The constructor was being called explicitly on the node
`value_` but the `next_` pointer remained uninitialized rather than
being set to null. This bug is only triggered in the cleanup code if an
exception is thrown -- upon successful creation of new nodes, the last
incorrect "next" value is overwritten to a correct pointer.
This issue was found due to new tests added in
https://reviews.llvm.org/D149830.
Differential Revision: https://reviews.llvm.org/D152327
