fix(api): make nil value in nvim_set_option_value clear local value

[gpanders]

Fix a bug in nvim_set_option_value that would set number and boolean options
to 0 or false respectively when passing nil as the value. A value of
nil should instead clear the local value, which has different meanings for
different option types:

String options: set the local value to the empty string ("") which has the
effect of using the global value of the option for global-local options.

Number and boolean options: set the local value to the global value.

TODO:

• Handle global-local number options (autoread, scrolloff, undolevels)

[bfredl]

Does this work for global-local bool/number options? I e autoread, undolevel and scrolloff should the sentinel value set, rather than copy from global value.

[gpanders]

Does this work for global-local bool/number options? I e autoread, undolevel and scrolloff should the sentinel value set, rather than copy from global value.

Thanks, forgot about these. These will need to be specially handled unfortunately, as they are in do_set right now.

[bfredl]

I'd suggest using NIL to represent unset autoread value.

[gpanders]

-1 is used in do_set():

neovim/src/nvim/option.c

Lines 1211 to 1218 in abb2b1a

	// For 'autoread' -1 means to use global value.
	if ((int *)varp == &curbuf->b_p_ar
	&& opt_flags == OPT_LOCAL) {
	value = -1;
	} else {
	value = *(int *)get_varp_scope(&(options[opt_idx]),
	OPT_GLOBAL);
	}

:set autoread
:setl autoread<
:echo &l:autoread
-1

Shouldn't we try and keep the API version consistent with the Vim script version?

[bfredl]

what congruence? &l:autoread always returns an integer value, never a bool, so already using booleans breaks that rule in that case. vim script cannot change it to neither v:true / v:false nor v:null due to backwards compatibility, while I think API functions should use API types whenever possible/reasonable.

[gpanders]

Ok done.

[tjdevries]

yes please, if we can make the API return bools instead of ints, that would be GREAT for the common case of:

if not vim.opt.xyz:get() then do_something() end

where xyz always returns 0 or 1.... but that's never falsey in lua.

[github-actions]

Backport failed for release-0.6, because it was unable to cherry-pick the commit(s).

Please cherry-pick the changes locally.

git fetch origin release-0.6
git worktree add -d .worktree/backport-16710-to-release-0.6 origin/release-0.6
cd .worktree/backport-16710-to-release-0.6
git checkout -b backport-16710-to-release-0.6
ancref=$(git merge-base b42e0c40c8b8e906eccb14db0d0648a39e49c363 e5e7834e88db9fecad00565fb59ba1a6a7cc1c8b)
git cherry-pick -x $ancref..e5e7834e88db9fecad00565fb59ba1a6a7cc1c8b

[gpanders]

Ah, this shouldn't be backported since #15996 was not backported.