vim-patch:partial:8.2.3908: cannot use a script-local function for 'foldtext'

Problem:    Cannot use a script-local function for 'foldtext'.
Solution:   Expand "s:" and "<SID>". (Yegappan Lakshmanan, closes vim/vim#9411)

https://github.com/vim/vim/commit/27708e6c7b6f444fd599f3dc5015336b002b874d

Only port the changes actually related to 'foldtext'.

Co-authored-by: Yegappan Lakshmanan <yegappan@yahoo.com>

3 files changed, 41 insertions, 1 deletions

diff --git a/runtime/doc/fold.txt b/runtime/doc/fold.txt
index 04c5b70c45..fa823608df 100644
--- a/runtime/doc/fold.txt
+++ b/runtime/doc/fold.txt
@@ -117,7 +117,7 @@ Try to avoid the "=", "a" and "s" return values, since Vim often has to search
 backwards for a line for which the fold level is defined.  This can be slow.
 
 If the 'foldexpr' expression starts with s: or |<SID>|, then it is replaced
-with the script ID (|local-function|). Example: >
+with the script ID (|local-function|). Examples: >
 		set foldexpr=s:MyFoldExpr()
 		set foldexpr=<SID>SomeFoldExpr()
 <
@@ -526,6 +526,11 @@ The resulting line is truncated to fit in the window, it never wraps.
 When there is room after the text, it is filled with the character specified
 by 'fillchars'.
 
+If the 'foldtext' expression starts with s: or |<SID>|, then it is replaced
+with the script ID (|local-function|). Examples: >
+		set foldtext=s:MyFoldText()
+		set foldtext=<SID>SomeFoldText()
+<
 Note that backslashes need to be used for characters that the ":set" command
 handles differently: Space, backslash and double-quote. |option-backslash|
 
diff --git a/src/nvim/optionstr.c b/src/nvim/optionstr.c
index b5dce7ff2a..cd87db9aa9 100644
--- a/src/nvim/optionstr.c
+++ b/src/nvim/optionstr.c
@@ -1483,6 +1483,7 @@ char *did_set_string_option(int opt_idx, char **varp, char *oldval, char *errbuf
     }
   } else if (varp == &p_dex
              || varp == &curwin->w_p_fde
+             || varp == &curwin->w_p_fdt
              || gvarp == &p_fex
              || gvarp == &p_inex
              || gvarp == &p_inde
@@ -1499,6 +1500,9 @@ char *did_set_string_option(int opt_idx, char **varp, char *oldval, char *errbuf
     if (varp == &curwin->w_p_fde) {  // 'foldexpr'
       p_opt = &curwin->w_p_fde;
     }
+    if (varp == &curwin->w_p_fdt) {  // 'foldtext'
+      p_opt = &curwin->w_p_fdt;
+    }
     if (gvarp == &p_fex) {  // 'formatexpr'
       p_opt = &curbuf->b_p_fex;
     }
diff --git a/src/nvim/testdir/test_fold.vim b/src/nvim/testdir/test_fold.vim
index bc8364a80e..d74187537c 100644
--- a/src/nvim/testdir/test_fold.vim
+++ b/src/nvim/testdir/test_fold.vim
@@ -1299,6 +1299,37 @@ func Test_foldexpr_scriptlocal_func()
   bw!
 endfunc
 
+" Test for using a script-local function for 'foldtext'
+func Test_foldtext_scriptlocal_func()
+  func! s:FoldText()
+    let g:FoldTextArgs = [v:foldstart, v:foldend]
+    return foldtext()
+  endfunc
+  new | only
+  call setline(1, range(50))
+  let g:FoldTextArgs = []
+  set foldmethod=manual
+  set foldtext=s:FoldText()
+  norm! 4Gzf4j
+  redraw!
+  call assert_equal(expand('<SID>') .. 'FoldText()', &foldtext)
+  call assert_equal([4, 8], g:FoldTextArgs)
+  set foldtext&
+  bw!
+  new | only
+  call setline(1, range(50))
+  let g:FoldTextArgs = []
+  set foldmethod=manual
+  set foldtext=<SID>FoldText()
+  norm! 8Gzf4j
+  redraw!
+  call assert_equal(expand('<SID>') .. 'FoldText()', &foldtext)
+  call assert_equal([8, 12], g:FoldTextArgs)
+  set foldtext&
+  bw!
+  delfunc s:FoldText
+endfunc
+
 " Make sure a fold containing a nested fold is split correctly when using
 " foldmethod=indent
 func Test_fold_split()