[vim/vim] :autocmd `<sfile>` behavior documented at `:h autocmd-expand` at doesn't always work (Issue #17569)

[zeertzjq]

zeertzjq created an issue (vim/vim#17569)

In :h autocmd-expand:

							*autocmd-expand*
Note that special characters (e.g., "%", "<cword>") in the ":autocmd"
arguments are not expanded when the autocommand is defined.  These will be
expanded when the Event is recognized, and the {cmd} is executed.  The only
exception is that "<sfile>" is expanded when the autocmd is defined.  Example:
>
	:au BufNewFile,BufRead *.html so <sfile>:h/html.vim

Here Vim expands <sfile> to the name of the file containing this line.

However, in :h <sfile>:

							*:<sfile>* *<sfile>*
	<sfile>    When executing a `:source` command, is replaced with the
		   file name of the sourced file.  *E498*
		   When executing a legacy function, is replaced with the call
		   stack, as with <stack> (this is for backwards
		   compatibility, using <stack> or <script> is preferred).
		   In Vim9 script using <sfile> in a function gives error
		   *E1245* .

And indeed, when sourcing the following file saved as /tmp/tmp.vim using vim --clean -S /tmp/tmp.vim:

func Foo()
  au BufNewFile,BufRead *.html so <sfile>:h/html.vim
endfunc
call Foo()

the defined autocommand becomes the call stack:

so command line..script /tmp/html.vim

while normally (with :source /tmp/tmp.vim) the autocommand is:

so /tmp/html.vim

What's more, using <sfile> in :autocmd isn't allowed in a :def function:

def Foo()
  au BufNewFile,BufRead *.html so <sfile>:h/html.vim
enddef
call Foo()

Changing <sfile> to <script> won't work either, as <script> is expanded when executing the autocommand, not when defining it.

I think there a 3 solutions to this problem:

1. Change the behavior of <sfile> in :autocmd to be the same as <script>.
2. Also expand <script> in :autocmd when defining the autocommand.
3. Don't change any behavior, but update the documentation to encourage using :execute with expand('<script>') (which is a bit verbose) instead of <sfile> in :autocmd.

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[Christian Brabandt]

chrisbra left a comment (vim/vim#17569)

Hm, not sure. Nobody has complained until yet.

1. Change the behavior of in :autocmd to be the same as <script>.

That would not help with vim9 script.

2. Also expand <script> in :autocmd when defining an autocommand rather than when executing it.

This would be a backwards incompatible change.

3. Don't change any behavior, but update the documentation to encourage using :execute with expand('<script>') (which is a bit verbose) instead of in :autocmd.

That makes sense and would need to be done anyhow for Vim9 script, no? Perhaps we can follow what you have done with Neovim and emphasis that <sfile> is deprecated and should not be used for new code (but don't wipe it yet from the documentation).

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[Christian Brabandt]

Closed #17569 as completed via 476b65e.

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