vim-patch:9.0.0457: substitute prompt does not highlight an empty match (#20186)

Problem:    Substitute prompt does not highlight an empty match.
Solution:   Highlight at least one character.
a04f457a6c

src/nvim/ex_cmds.c

@@ -3887,6 +3887,10 @@ static int do_sub(exarg_T *eap, proftime_T timeout, long cmdpreview_ns, handle_T
                                    - regmatch.startpos[0].lnum;
               search_match_endcol = regmatch.endpos[0].col
                                     + len_change;
+              if (search_match_lines == 0 && search_match_endcol == 0) {
+                // highlight at least one character for /^/
+                search_match_endcol = 1;
+              }
               highlight_match = true;
 
               update_topline(curwin);

src/nvim/testdir/test_substitute.vim

@@ -1,6 +1,8 @@
 " Tests for the substitute (:s) command
 
 source shared.vim
+source check.vim
+source screendump.vim
 
 func Test_multiline_subst()
   enew!
@@ -668,6 +670,21 @@ func Test_sub_cmd_9()
   bw!
 endfunc
 
+func Test_sub_highlight_zero_match()
+  CheckRunVimInTerminal
+
+  let lines =<< trim END
+    call setline(1, ['one', 'two', 'three'])
+  END
+  call writefile(lines, 'XscriptSubHighlight', 'D')
+  let buf = RunVimInTerminal('-S XscriptSubHighlight', #{rows: 8, cols: 60})
+  call term_sendkeys(buf, ":%s/^/   /c\<CR>")
+  call VerifyScreenDump(buf, 'Test_sub_highlight_zer_match_1', {})
+
+  call term_sendkeys(buf, "\<Esc>")
+  call StopVimInTerminal(buf)
+endfunc
+
 func Test_nocatch_sub_failure_handling()
   " normal error results in all replacements 
   func Foo()

test/functional/legacy/substitute_spec.lua

@@ -3,11 +3,13 @@
 -- Test for *:s%* on :substitute.
 
 local helpers = require('test.functional.helpers')(after_each)
+local Screen = require('test.functional.ui.screen')
 local feed, insert = helpers.feed, helpers.insert
+local exec = helpers.exec
 local clear, feed_command, expect = helpers.clear, helpers.feed_command, helpers.expect
 local eq, eval = helpers.eq, helpers.eval
 
-describe('substitue()', function()
+describe('substitute()', function()
   before_each(clear)
 
   -- The original test contained several TEST_X lines to delimit different
@@ -132,7 +134,7 @@ describe('substitue()', function()
   end)
 end)
 
-describe(':substitue', function()
+describe(':substitute', function()
   before_each(clear)
 
   it('with \\ze and \\zs and confirmation dialog (TEST_8)', function()
@@ -159,4 +161,29 @@ describe(':substitue', function()
     feed('yyq')  -- For the dialog of the previous :s command.
     expect('XXx')
   end)
+
+  it('first char is highlighted with confirmation dialog and empty match', function()
+    local screen = Screen.new(60, 8)
+    screen:set_default_attr_ids({
+      [0] = {bold = true, foreground = Screen.colors.Blue},  -- NonText
+      [1] = {reverse = true},  -- IncSearch
+      [2] = {bold = true, foreground = Screen.colors.SeaGreen},  -- MoreMsg
+    })
+    screen:attach()
+    exec([[
+      set nohlsearch noincsearch
+      call setline(1, ['one', 'two', 'three'])
+    ]])
+    feed(':%s/^/   /c<CR>')
+    screen:expect([[
+      {1:o}ne                                                         |
+      two                                                         |
+      three                                                       |
+      {0:~                                                           }|
+      {0:~                                                           }|
+      {0:~                                                           }|
+      {0:~                                                           }|
+      {2:replace with     (y/n/a/q/l/^E/^Y)?}^                         |
+    ]])
+  end)
 end)