(mini.ai) Allow empty regions to work with 'mini.operators'.

Details:
- Resolves #441.

Author: echasnovski

lua/mini/ai.lua

@@ -987,8 +987,14 @@ MiniAi.select_textobject = function(ai_type, id, opts)
   local cache_eventignore = vim.o.eventignore
 
   pcall(function()
-    -- Do nothing in Operator-pending mode for empty region (except `c` or `d`)
-    if tobj_is_empty and opts.operator_pending and not (vim.v.operator == 'c' or vim.v.operator == 'd') then
+    -- Do nothing in Operator-pending mode for empty region (except `c`, `d`,
+    -- or "replace" from 'mini.operators'). These are hand picked because they
+    -- completely remove selected text, which is necessary for currently only
+    -- possible empty region selection implementation.
+    local is_empty_opending = tobj_is_empty and opts.operator_pending
+    local is_minioperators_replace = vim.v.operator == 'g@' and vim.o.operatorfunc:find('MiniOperators%.replace') ~= nil
+    local is_allowed_empty_opending = vim.v.operator == 'c' or vim.v.operator == 'd' or is_minioperators_replace
+    if is_empty_opending and not is_allowed_empty_opending then
       H.message('Textobject region is empty. Nothing is done.')
       return
     end
@@ -1009,7 +1015,7 @@ MiniAi.select_textobject = function(ai_type, id, opts)
     vim.cmd('normal! ' .. vis_mode)
     set_cursor(tobj.from)
 
-    if tobj_is_empty and opts.operator_pending then
+    if is_empty_opending then
       -- Add single space (without triggering events) and visually select it.
       -- Seems like the only way to make `ci)` and `di)` move inside empty
       -- brackets. Original idea is from 'wellle/targets.vim'.

tests/test_ai.lua

@@ -2042,13 +2042,31 @@ T['Builtin']['Bracket']['is balanced'] = function(key)
 end
 
 T['Builtin']['Bracket']['works with empty region'] = function(key)
+  -- Emulate "replace" from 'mini.operators'
+  child.lua([[
+    _G.MiniOperators = {}
+    _G.MiniOperators.replace = function(mode)
+      if mode == nil then
+        vim.o.operatorfunc = 'v:lua.MiniOperators.replace'
+        return 'g@'
+      end
+      vim.fn.setreg('a', 'xxx')
+      vim.cmd('normal! `[v`]"aP`[')
+    end
+
+    vim.keymap.set('n', 'gr', _G.MiniOperators.replace, { expr = true })
+  ]])
+
+  -- Validate
   local left, right, _ = unpack(brackets[key])
 
   local line = 'a' .. left .. right
   for i = 0, 1 do
     validate_tobj1d(line, i, { 'i', key }, { 3, 3 })
     validate_edit1d(line, i, line, 2, { 'ci', key })
     validate_edit1d(line, i, line, 2, { 'di', key })
+
+    validate_edit1d(line, i, 'a' .. left .. 'xxx' .. right, 2, { 'gri', key })
   end
 end