vim-patch:8.2.5034: there is no way to get the byte index from a virtual column

Problem:    There is no way to get the byte index from a virtual column.
Solution:   Add virtcol2col(). (Yegappan Lakshmanan, closes vim/vim#10477,
            closes vim/vim#10098)
https://github.com/vim/vim/commit/5a6ec10cc80ab02eeff644ab19b82312630ea855

Cherry-pick tv_check_for_number_arg() from Vim.
Cherry-pick pathshorten() doc change.

4 files changed, 68 insertions, 0 deletions

diff --git a/src/nvim/eval.lua b/src/nvim/eval.lua
index 3e89489459..5c500af899 100644
--- a/src/nvim/eval.lua
+++ b/src/nvim/eval.lua
@@ -425,6 +425,7 @@ return {
     uniq={args={1, 3}, base=1},
     values={args=1, base=1},
     virtcol={args=1, base=1},
+    virtcol2col={args=3, base=1},
     visualmode={args={0, 1}},
     wait={args={2,3}},
     wildmenumode={},
diff --git a/src/nvim/eval/typval.c b/src/nvim/eval/typval.c
index c75d16e4fc..2089415ffa 100644
--- a/src/nvim/eval/typval.c
+++ b/src/nvim/eval/typval.c
@@ -44,6 +44,8 @@ static char e_string_required_for_argument_nr[]
   = N_("E1174: String required for argument %d");
 static char e_non_empty_string_required_for_argument_nr[]
   = N_("E1142: Non-empty string required for argument %d");
+static char e_number_required_for_argument_nr[]
+  = N_("E1210: Number required for argument %d");
 
 bool tv_in_free_unref_items = false;
 
@@ -3830,6 +3832,17 @@ int tv_check_for_nonempty_string_arg(const typval_T *const args, const int idx)
   return OK;
 }
 
+/// Give an error and return FAIL unless "args[idx]" is a number.
+int tv_check_for_number_arg(const typval_T *const args, const int idx)
+  FUNC_ATTR_NONNULL_ALL FUNC_ATTR_WARN_UNUSED_RESULT FUNC_ATTR_PURE
+{
+  if (args[idx].v_type != VAR_NUMBER) {
+    semsg(_(e_number_required_for_argument_nr), idx + 1);
+    return FAIL;
+  }
+  return OK;
+}
+
 /// Get the string value of a "stringish" VimL object.
 ///
 /// @param[in]  tv  Object to get value of.
diff --git a/src/nvim/move.c b/src/nvim/move.c
index 9e0662c141..481746881b 100644
--- a/src/nvim/move.c
+++ b/src/nvim/move.c
@@ -22,12 +22,14 @@
 #include "nvim/drawscreen.h"
 #include "nvim/edit.h"
 #include "nvim/eval.h"
+#include "nvim/eval/typval.h"
 #include "nvim/fold.h"
 #include "nvim/getchar.h"
 #include "nvim/grid.h"
 #include "nvim/highlight.h"
 #include "nvim/mbyte.h"
 #include "nvim/memline.h"
+#include "nvim/mouse.h"
 #include "nvim/move.h"
 #include "nvim/option.h"
 #include "nvim/plines.h"
@@ -1004,6 +1006,36 @@ void f_screenpos(typval_T *argvars, typval_T *rettv, EvalFuncData fptr)
   tv_dict_add_nr(dict, S_LEN("endcol"), ecol);
 }
 
+/// "virtcol2col({winid}, {lnum}, {col})" function
+void f_virtcol2col(typval_T *argvars, typval_T *rettv, EvalFuncData fptr)
+{
+  rettv->vval.v_number = -1;
+
+  if (tv_check_for_number_arg(argvars, 0) == FAIL
+      || tv_check_for_number_arg(argvars, 1) == FAIL
+      || tv_check_for_number_arg(argvars, 2) == FAIL) {
+    return;
+  }
+
+  win_T *wp = find_win_by_nr_or_id(&argvars[0]);
+  if (wp == NULL) {
+    return;
+  }
+
+  bool error = false;
+  linenr_T lnum = (linenr_T)tv_get_number_chk(&argvars[1], &error);
+  if (error || lnum < 0 || lnum > wp->w_buffer->b_ml.ml_line_count) {
+    return;
+  }
+
+  int screencol = (int)tv_get_number_chk(&argvars[2], &error);
+  if (error || screencol < 0) {
+    return;
+  }
+
+  rettv->vval.v_number = vcol2col(wp, lnum, screencol);
+}
+
 /// Scroll the current window down by "line_count" logical lines.  "CTRL-Y"
 ///
 /// @param line_count number of lines to scroll
diff --git a/src/nvim/testdir/test_cursor_func.vim b/src/nvim/testdir/test_cursor_func.vim
index f13842edc8..2e625f2388 100644
--- a/src/nvim/testdir/test_cursor_func.vim
+++ b/src/nvim/testdir/test_cursor_func.vim
@@ -373,4 +373,26 @@ func Test_setcursorcharpos()
   %bw!
 endfunc
 
+" Test for virtcol2col()
+func Test_virtcol2col()
+  new
+  call setline(1, ["a\tb\tc"])
+  call assert_equal(1, virtcol2col(0, 1, 1))
+  call assert_equal(2, virtcol2col(0, 1, 2))
+  call assert_equal(2, virtcol2col(0, 1, 8))
+  call assert_equal(3, virtcol2col(0, 1, 9))
+  call assert_equal(4, virtcol2col(0, 1, 10))
+  call assert_equal(4, virtcol2col(0, 1, 16))
+  call assert_equal(5, virtcol2col(0, 1, 17))
+  call assert_equal(-1, virtcol2col(10, 1, 1))
+  call assert_equal(-1, virtcol2col(0, 10, 1))
+  call assert_equal(-1, virtcol2col(0, -1, 1))
+  call assert_equal(-1, virtcol2col(0, 1, -1))
+  call assert_equal(5, virtcol2col(0, 1, 20))
+  call assert_fails('echo virtcol2col("0", 1, 20)', 'E1210:')
+  call assert_fails('echo virtcol2col(0, "1", 20)', 'E1210:')
+  call assert_fails('echo virtcol2col(0, 1, "1")', 'E1210:')
+  bw!
+endfunc
+
 " vim: shiftwidth=2 sts=2 expandtab