:undo! doesn't treat number as documented

[echasnovski]

Describe the bug

Executing :undo! when not on latest change restores previous to target undo state.

Steps to reproduce

1. nvim --clean -u NONE.
2. Create undo history by typing:
   2.1. A, one, <Esc>.
   2.2. A, two, <Esc>.
   2.3. A, three, <Esc>.
   2.4. A, four, <Esc>.
   2.5. A, five, <Esc>.
3. Press u. This will restore one two three four text in undo state 4.
4. Execute :undo! 3. It should restore one two three text in undo state 3, but restores one two text in undo state 2. Executing :echo undotree() confirms that states 3, 4, 5 are deleted.

Expected behavior

:undo! always restores target undo state while deleting all states after it.

Notes:

• When step 3 is skipped and :undo! 3 is executed on latest undo state, everything works as expected.

Neovim version (nvim -v)

NVIM v0.9.0-dev-987+g371a74e4e

Vim (not Nvim) behaves the same?

No (doesn't have this functionality)

Operating system/version

EndeavourOS Linux x86_64 (6.1.11-arch1-1)

Terminal name/version

st-0.9

$TERM environment variable

st-256color

Installation

appimage

[zeertzjq]

The docs of :undo! seems wrong. The {N} has complete different meanings in :undo! {N} and :undo {N}.

[zeertzjq]

Although the behavior does seem a bit strange...