vim-patch:9.1.1797: completion: autocompletion can be improved

Problem:  completion: autocompletion can be improved
Solution: Add support for "longest" and "preinsert" in 'autocomplete';
          add preinserted() (Girish Palya)

* Add support for "longest" in 'completeopt' when 'autocomplete'
  is enabled. (Note: the cursor position does not change automatically
  when 'autocomplete' is enabled.)
* Add support for "preinsert" when 'autocomplete' is enabled. Ensure
  "preinsert" works the same with and without 'autocomplete'
* introduce the preinserted() Vim script function, useful for defining
  custom key mappings.

fixes: vim/vim#18314
closes: vim/vim#18387

c05335082a

Co-authored-by: Girish Palya <girishji@gmail.com>

runtime/doc/options.txt

@@ -1638,11 +1638,22 @@ A jump table for the options with a short description can be found at |Q_op|.
 		    to gather more alternatives for your candidate list,
 		    see 'completefuzzycollect'.
 
-	   longest  Only insert the longest common text of the matches.  If
+	   longest
-		    the menu is displayed you can use CTRL-L to add more
+		    When 'autocomplete' is not active, only the longest
-		    characters.  Whether case is ignored depends on the kind
+		    common prefix of the matches is inserted.  If the popup
-		    of completion.  For buffer text the 'ignorecase' option is
+		    menu is displayed, you can use CTRL-L to add more
-		    used.
+		    characters.  Whether case is ignored depends on the type
+		    of completion.  For buffer text the 'ignorecase' option
+		    applies.
+
+		    When 'autocomplete' is active and no completion item is
+		    selected, the longest common prefix of the matches is
+		    inserted after the cursor.  The prefix is taken either
+		    from all displayed items or only from items in the current
+		    buffer.  The inserted text is highlighted with
+		    |hl-PreInsert|, and the cursor position does not change
+		    (similar to `"preinsert"`).  Press CTRL-Y to accept.
+		    See also |preinserted()|.
 
 	   menu	    Use a popup menu to show the possible completions.  The
 		    menu is only shown when there is more than one match and
@@ -1675,22 +1686,21 @@ A jump table for the options with a short description can be found at |Q_op|.
 		    with "menu" or "menuone".  Overrides "preview".
 
 	   preinsert
-		    When 'autocomplete' is not active, inserts the part of the
+		    Inserts the text of the first completion candidate
-		    first candidate word beyond the current completion leader,
+		    beyond the current leader, highlighted with |hl-PreInsert|.
-		    highlighted with |hl-PreInsert|.  The cursor doesn't move.
+		    The cursor does not move.
-		    Requires "fuzzy" unset and "menuone" in 'completeopt'.
+		    Requires "fuzzy" to be unset, and either "menuone" in
-
+		    'completeopt' or 'autocomplete' enabled.  When
-		    When 'autocomplete' is active, inserts the longest common
+		    'autocomplete' is enabled, this does not work if
-		    prefix of matches (from all shown items or from the
+		    'ignorecase' is set without 'infercase'.
-		    current buffer items).  This occurs only when no menu item
+		    See also |preinserted()|.
-		    is selected.  Press CTRL-Y to accept.
 
 	   preview  Show extra information about the currently selected
 		    completion in the preview window.  Only works in
 		    combination with "menu" or "menuone".
 
-	Only "fuzzy", "popup", "preinsert" and "preview" have an effect when
+	Only "fuzzy", "longest", "popup", "preinsert" and "preview" have an
-	'autocomplete' is enabled.
+	effect when 'autocomplete' is enabled.
 
 	This option does not apply to |cmdline-completion|.  See 'wildoptions'
 	for that.

runtime/doc/usr_41.txt

@@ -940,6 +940,7 @@ Insert mode completion:				*completion-functions*
 	complete_info()		get current completion information
 	complete_match()	get insert completion start match col and
 				trigger text
+	preinserted()		check if text is inserted after cursor
 	pumvisible()		check if the popup menu is displayed
 	pum_getpos()		position and size of popup menu if visible
 

runtime/doc/vimfn.txt

@@ -7232,6 +7232,15 @@ pow({x}, {y})                                                            *pow()*
                 Return: ~
                   (`number`)
 
+preinserted()                                                    *preinserted()*
+		Returns non-zero if text has been inserted after the cursor
+		because "preinsert" is present in 'completeopt', or if
+		"longest" is present in 'completeopt' while 'autocomplete'
+		is enabled.  Otherwise returns zero.
+
+                Return: ~
+                  (`number`)
+
 prevnonblank({lnum})                                            *prevnonblank()*
 		Return the line number of the first line at or above {lnum}
 		that is not blank.  Example: >vim

runtime/lua/vim/_meta/options.lua

@@ -1183,11 +1183,22 @@ vim.go.cia = vim.go.completeitemalign
 --- 	    to gather more alternatives for your candidate list,
 --- 	    see 'completefuzzycollect'.
 ---
----    longest  Only insert the longest common text of the matches.  If
+---    longest
---- 	    the menu is displayed you can use CTRL-L to add more
+--- 	    When 'autocomplete' is not active, only the longest
---- 	    characters.  Whether case is ignored depends on the kind
+--- 	    common prefix of the matches is inserted.  If the popup
---- 	    of completion.  For buffer text the 'ignorecase' option is
+--- 	    menu is displayed, you can use CTRL-L to add more
---- 	    used.
+--- 	    characters.  Whether case is ignored depends on the type
+--- 	    of completion.  For buffer text the 'ignorecase' option
+--- 	    applies.
+---
+--- 	    When 'autocomplete' is active and no completion item is
+--- 	    selected, the longest common prefix of the matches is
+--- 	    inserted after the cursor.  The prefix is taken either
+--- 	    from all displayed items or only from items in the current
+--- 	    buffer.  The inserted text is highlighted with
+--- 	    `hl-PreInsert`, and the cursor position does not change
+--- 	    (similar to `"preinsert"`).  Press CTRL-Y to accept.
+--- 	    See also `preinserted()`.
 ---
 ---    menu	    Use a popup menu to show the possible completions.  The
 --- 	    menu is only shown when there is more than one match and
@@ -1220,22 +1231,21 @@ vim.go.cia = vim.go.completeitemalign
 --- 	    with "menu" or "menuone".  Overrides "preview".
 ---
 ---    preinsert
---- 	    When 'autocomplete' is not active, inserts the part of the
+--- 	    Inserts the text of the first completion candidate
---- 	    first candidate word beyond the current completion leader,
+--- 	    beyond the current leader, highlighted with `hl-PreInsert`.
---- 	    highlighted with `hl-PreInsert`.  The cursor doesn't move.
+--- 	    The cursor does not move.
---- 	    Requires "fuzzy" unset and "menuone" in 'completeopt'.
+--- 	    Requires "fuzzy" to be unset, and either "menuone" in
----
+--- 	    'completeopt' or 'autocomplete' enabled.  When
---- 	    When 'autocomplete' is active, inserts the longest common
+--- 	    'autocomplete' is enabled, this does not work if
---- 	    prefix of matches (from all shown items or from the
+--- 	    'ignorecase' is set without 'infercase'.
---- 	    current buffer items).  This occurs only when no menu item
+--- 	    See also `preinserted()`.
---- 	    is selected.  Press CTRL-Y to accept.
 ---
 ---    preview  Show extra information about the currently selected
 --- 	    completion in the preview window.  Only works in
 --- 	    combination with "menu" or "menuone".
 ---
---- Only "fuzzy", "popup", "preinsert" and "preview" have an effect when
+--- Only "fuzzy", "longest", "popup", "preinsert" and "preview" have an
---- 'autocomplete' is enabled.
+--- effect when 'autocomplete' is enabled.
 ---
 --- This option does not apply to `cmdline-completion`.  See 'wildoptions'
 --- for that.

runtime/lua/vim/_meta/vimfn.lua

@@ -6554,6 +6554,14 @@ function vim.fn.perleval(expr) end
 --- @return number
 function vim.fn.pow(x, y) end
 
+--- Returns non-zero if text has been inserted after the cursor
+--- because "preinsert" is present in 'completeopt', or if
+--- "longest" is present in 'completeopt' while 'autocomplete'
+--- is enabled.  Otherwise returns zero.
+---
+--- @return number
+function vim.fn.preinserted() end
+
 --- Return the line number of the first line at or above {lnum}
 --- that is not blank.  Example: >vim
 ---   let ind = indent(prevnonblank(v:lnum - 1))

src/nvim/edit.c

@@ -606,10 +606,10 @@ static int insert_execute(VimState *state, int key)
                && (s->c == CAR || s->c == K_KENTER || s->c == NL)))
           && stop_arrow() == OK) {
         ins_compl_delete(false);
-        if (ins_compl_has_preinsert() && ins_compl_autocomplete_enabled()) {
+        ins_compl_insert(false, !ins_compl_has_preinsert());
-          (void)ins_compl_insert(false, true);
+        if (ins_compl_preinsert_longest()) {
-        } else {
+          ins_compl_init_get_longest();
-          (void)ins_compl_insert(false, false);
+          return 1;
         }
       } else if (ascii_iswhite_nl_or_nul(s->c) && ins_compl_preinsert_effect()) {
         // Delete preinserted text when typing special chars

src/nvim/eval.lua

@@ -8024,6 +8024,18 @@ M.funcs = {
     returns = 'number',
     signature = 'pow({x}, {y})',
   },
+  preinserted = {
+    desc = [=[
+      Returns non-zero if text has been inserted after the cursor
+      because "preinsert" is present in 'completeopt', or if
+      "longest" is present in 'completeopt' while 'autocomplete'
+      is enabled.  Otherwise returns zero.
+    ]=],
+    name = 'preinserted',
+    params = {},
+    returns = 'number',
+    signature = 'preinserted()',
+  },
   prevnonblank = {
     args = 1,
     base = 1,

src/nvim/insexpand.c

@@ -296,7 +296,7 @@ static bool compl_autocomplete = false;        ///< whether autocompletion is ac
 static uint64_t compl_timeout_ms = COMPL_INITIAL_TIMEOUT_MS;
 static bool compl_time_slice_expired = false;  ///< time budget exceeded for current source
 static bool compl_from_nonkeyword = false;     ///< completion started from non-keyword
-static bool compl_autocomplete_preinsert = false;  ///< apply preinsert highlight
+static bool compl_hi_on_autocompl_longest = false;  ///< apply "PreInsert" highlight
 
 // Halve the current completion timeout, simulating exponential decay.
 #define COMPL_MIN_TIMEOUT_MS    5
@@ -889,6 +889,15 @@ static bool is_nearest_active(void)
          && !(flags & kOptCotFlagFuzzy);
 }
 
+/// Returns true if autocomplete is active and the pre-insert effect targets the
+/// longest prefix.
+bool ins_compl_preinsert_longest(void)
+{
+  return compl_autocomplete
+         && (get_cot_flags() & (kOptCotFlagLongest | kOptCotFlagPreinsert | kOptCotFlagFuzzy))
+         == kOptCotFlagLongest;
+}
+
 /// Add a match to the list of matches
 ///
 /// @param[in]  str     text of the match to add
@@ -1053,7 +1062,8 @@ static int ins_compl_add(char *const str, int len, char *const fname, char *cons
   compl_curr_match = match;
 
   // Find the longest common string if still doing that.
-  if (compl_get_longest && (flags & CP_ORIGINAL_TEXT) == 0 && !cfc_has_mode()) {
+  if (compl_get_longest && (flags & CP_ORIGINAL_TEXT) == 0 && !cfc_has_mode()
+      && !ins_compl_preinsert_longest()) {
     ins_compl_longest_match(match);
   }
 
@@ -1107,17 +1117,12 @@ static size_t ins_compl_leader_len(void)
 /// -1 means normal item.
 int ins_compl_col_range_attr(linenr_T lnum, int col)
 {
-  const bool has_preinsert = ins_compl_has_preinsert();
+  const bool has_preinsert = ins_compl_has_preinsert() || ins_compl_preinsert_longest();
 
   int attr;
   if ((get_cot_flags() & kOptCotFlagFuzzy)
-      || (!has_preinsert
+      || (!compl_hi_on_autocompl_longest && ins_compl_preinsert_longest())
-          && (attr = syn_name2attr("ComplMatchIns")) == 0)
+      || (attr = syn_name2attr(has_preinsert ? "PreInsert" : "ComplMatchIns")) == 0) {
-      || (!compl_autocomplete && has_preinsert
-          && (attr = syn_name2attr("PreInsert")) == 0)
-      || (compl_autocomplete
-          && (!compl_autocomplete_preinsert
-              || (attr = syn_name2attr("PreInsert")) == 0))) {
     return -1;
   }
 
@@ -1511,7 +1516,8 @@ static int ins_compl_build_pum(void)
   }
 
   unsigned cur_cot_flags = get_cot_flags();
-  bool compl_no_select = (cur_cot_flags & kOptCotFlagNoselect) != 0 || compl_autocomplete;
+  bool compl_no_select = (cur_cot_flags & kOptCotFlagNoselect) != 0
+                         || (compl_autocomplete && !ins_compl_has_preinsert());
   bool fuzzy_filter = (cur_cot_flags & kOptCotFlagFuzzy) != 0;
 
   compl_T *match_head = NULL, *match_tail = NULL;
@@ -2132,6 +2138,9 @@ int ins_compl_len(void)
 bool ins_compl_has_preinsert(void)
 {
   unsigned cur_cot_flags = get_cot_flags();
+  if (compl_autocomplete && p_ic && !p_inf) {
+    return false;
+  }
   return (!compl_autocomplete
           ? (cur_cot_flags & (kOptCotFlagPreinsert|kOptCotFlagFuzzy|kOptCotFlagMenuone))
           == (kOptCotFlagPreinsert|kOptCotFlagMenuone)
@@ -2143,19 +2152,13 @@ bool ins_compl_has_preinsert(void)
 /// the `compl_ins_end_col` range.
 bool ins_compl_preinsert_effect(void)
 {
-  if (!ins_compl_has_preinsert()) {
+  if (!ins_compl_has_preinsert() && !ins_compl_preinsert_longest()) {
     return false;
   }
 
   return curwin->w_cursor.col < compl_ins_end_col;
 }
 
-/// Returns true if autocompletion is active.
-bool ins_compl_autocomplete_enabled(void)
-{
-  return compl_autocomplete;
-}
-
 /// Delete one character before the cursor and show the subset of the matches
 /// that match the word that is now before the cursor.
 /// Returns the character to be used, NUL if the work is done and another char
@@ -2198,7 +2201,7 @@ int ins_compl_bs(void)
                                 (size_t)(p_off - (ptrdiff_t)compl_col));
 
   // Clear selection if a menu item is currently selected in autocompletion
-  if (compl_autocomplete && compl_first_match) {
+  if (compl_autocomplete && compl_first_match && !ins_compl_has_preinsert()) {
     compl_shown_match = compl_first_match;
   }
 
@@ -2291,18 +2294,14 @@ static void ins_compl_new_leader(void)
   // Show the popup menu with a different set of matches.
   ins_compl_show_pum();
 
-  compl_autocomplete_preinsert = false;
   // Don't let Enter select the original text when there is no popup menu.
   if (compl_match_array == NULL) {
     compl_enter_selects = false;
   } else if (ins_compl_has_preinsert() && compl_leader.size > 0) {
-    if (compl_started && compl_autocomplete && !ins_compl_preinsert_effect()) {
+    ins_compl_insert(true, false);
-      if (ins_compl_insert(true, true) == OK) {
+  } else if (compl_started && ins_compl_preinsert_longest()
-        compl_autocomplete_preinsert = true;
+             && compl_leader.size > 0 && !ins_compl_preinsert_effect()) {
-      }
+    ins_compl_insert(true, true);
-    } else {
-      (void)ins_compl_insert(true, false);
-    }
   }
   // Don't let Enter select when use user function and refresh_always is set
   if (ins_compl_refresh_always()) {
@@ -4388,7 +4387,7 @@ static int get_next_default_completion(ins_compl_next_state_T *st, pos_T *start_
       ptr = ins_compl_get_next_word_or_line(st->ins_buf,
                                             st->cur_match_pos, &len, &cont_s_ipos);
     }
-    if (ptr == NULL || (!compl_autocomplete && ins_compl_has_preinsert()
+    if (ptr == NULL || (ins_compl_has_preinsert()
                         && strcmp(ptr, compl_pattern.data) == 0)) {
       continue;
     }
@@ -4895,7 +4894,7 @@ static int ins_compl_get_exp(pos_T *ini)
   }
   may_trigger_modechanged();
 
-  if (is_nearest_active()) {
+  if (is_nearest_active() && !ins_compl_has_preinsert()) {
     sort_compl_match_list(cp_compare_nearest);
   }
 
@@ -5108,6 +5107,21 @@ static char *find_common_prefix(size_t *prefix_len, bool curbuf_only)
   xfree(match_count);
 
   if (len > (int)ins_compl_leader_len()) {
+    assert(first != NULL);
+    // Avoid inserting text that duplicates the text already present
+    // after the cursor.
+    if (len == (int)strlen(first)) {
+      char *line = get_cursor_line_ptr();
+      char *p = line + curwin->w_cursor.col;
+      if (p && !ascii_iswhite_or_nul(*p)) {
+        char *end = find_word_end(p);
+        int text_len = (int)(end - p);
+        if (text_len > 0 && text_len < (len - (int)ins_compl_leader_len())
+            && strncmp(first + len - text_len, p, (size_t)text_len) == 0) {
+          len -= text_len;
+        }
+      }
+    }
     *prefix_len = (size_t)len;
     return first;
   }
@@ -5117,9 +5131,9 @@ static char *find_common_prefix(size_t *prefix_len, bool curbuf_only)
 /// Insert the new text being completed.
 /// "move_cursor" is used when 'completeopt' includes "preinsert" and when true
 /// cursor needs to move back from the inserted text to the compl_leader.
-/// When "preinsert_prefix" is true the longest common prefix is inserted
+/// When "insert_prefix" is true the longest common prefix is inserted instead
-/// instead of shown match.
+/// of shown match.
-int ins_compl_insert(bool move_cursor, bool preinsert_prefix)
+void ins_compl_insert(bool move_cursor, bool insert_prefix)
 {
   int compl_len = get_compl_len();
   bool preinsert = ins_compl_has_preinsert();
@@ -5128,12 +5142,13 @@ int ins_compl_insert(bool move_cursor, bool preinsert_prefix)
   size_t leader_len = ins_compl_leader_len();
   char *has_multiple = strchr(cp_str, '\n');
 
-  if (preinsert_prefix) {
+  if (insert_prefix) {
     cp_str = find_common_prefix(&cp_str_len, false);
     if (cp_str == NULL) {
       cp_str = find_common_prefix(&cp_str_len, true);
       if (cp_str == NULL) {
-        return FAIL;
+        cp_str = compl_shown_match->cp_str.data;
+        cp_str_len = compl_shown_match->cp_str.size;
       }
     }
   } else if (cpt_sources_array != NULL) {
@@ -5160,18 +5175,18 @@ int ins_compl_insert(bool move_cursor, bool preinsert_prefix)
       ins_compl_expand_multiple(cp_str + compl_len);
     } else {
       ins_compl_insert_bytes(cp_str + compl_len,
-                             preinsert_prefix ? (int)cp_str_len - compl_len : -1);
+                             insert_prefix ? (int)cp_str_len - compl_len : -1);
-      if (preinsert && move_cursor) {
+      if ((preinsert || insert_prefix) && move_cursor) {
         curwin->w_cursor.col -= (colnr_T)(cp_str_len - leader_len);
       }
     }
   }
   compl_used_match = !(match_at_original_text(compl_shown_match)
-                       || (preinsert && !compl_autocomplete));
+                       || (preinsert && !insert_prefix));
 
   dict_T *dict = ins_compl_dict_alloc(compl_shown_match);
   set_vim_var_dict(VV_COMPLETED_ITEM, dict);
-  return OK;
+  compl_hi_on_autocompl_longest = insert_prefix && move_cursor;
 }
 
 /// show the file name for the completion match (if any).  Truncate the file
@@ -5240,7 +5255,8 @@ static int find_next_completion_match(bool allow_get_expansion, int todo, bool a
   bool found_end = false;
   compl_T *found_compl = NULL;
   unsigned cur_cot_flags = get_cot_flags();
-  bool compl_no_select = (cur_cot_flags & kOptCotFlagNoselect) != 0 || compl_autocomplete;
+  bool compl_no_select = (cur_cot_flags & kOptCotFlagNoselect) != 0
+                         || (compl_autocomplete && !ins_compl_has_preinsert());
   bool compl_fuzzy_match = (cur_cot_flags & kOptCotFlagFuzzy) != 0;
 
   while (--todo >= 0) {
@@ -5351,7 +5367,8 @@ static int ins_compl_next(bool allow_get_expansion, int count, bool insert_match
   const bool started = compl_started;
   buf_T *const orig_curbuf = curbuf;
   unsigned cur_cot_flags = get_cot_flags();
-  bool compl_no_insert = (cur_cot_flags & kOptCotFlagNoinsert) != 0 || compl_autocomplete;
+  bool compl_no_insert = (cur_cot_flags & kOptCotFlagNoinsert) != 0
+                         || (compl_autocomplete && !ins_compl_has_preinsert());
   bool compl_fuzzy_match = (cur_cot_flags & kOptCotFlagFuzzy) != 0;
   bool compl_preinsert = ins_compl_has_preinsert();
 
@@ -5397,35 +5414,18 @@ static int ins_compl_next(bool allow_get_expansion, int count, bool insert_match
     return -1;
   }
 
-  compl_autocomplete_preinsert = false;
-
   // Insert the text of the new completion, or the compl_leader.
-  if (compl_no_insert && !started) {
+  if (!started && ins_compl_preinsert_longest()) {
-    bool insert_orig = !compl_preinsert;
+    ins_compl_insert(true, true);
-    if (compl_preinsert && compl_autocomplete) {
+  } else if (compl_no_insert && !started && !compl_preinsert) {
-      if (ins_compl_insert(true, true) == OK) {
+    ins_compl_insert_bytes(compl_orig_text.data + get_compl_len(), -1);
-        compl_autocomplete_preinsert = true;
-      } else {
-        insert_orig = true;
-      }
-    }
-    if (insert_orig) {
-      ins_compl_insert_bytes(compl_orig_text.data + get_compl_len(), -1);
-    }
     compl_used_match = false;
     restore_orig_extmarks();
   } else if (insert_match) {
     if (!compl_get_longest || compl_used_match) {
-      bool none_selected = match_at_original_text(compl_shown_match);
+      bool preinsert_longest = ins_compl_preinsert_longest()
-      if (compl_preinsert && compl_autocomplete && none_selected) {
+                               && match_at_original_text(compl_shown_match);  // none selected
-        if (ins_compl_insert(none_selected, true) == OK) {
+      ins_compl_insert(compl_preinsert || preinsert_longest, preinsert_longest);
-          compl_autocomplete_preinsert = none_selected;
-        } else {
-          (void)ins_compl_insert(false, false);
-        }
-      } else {
-        (void)ins_compl_insert(!compl_autocomplete, false);
-      }
     } else {
       assert(compl_leader.data != NULL);
       ins_compl_insert_bytes(compl_leader.data + get_compl_len(), -1);
@@ -5534,8 +5534,8 @@ void ins_compl_check_keys(int frequency, bool in_compl_func)
     }
   }
 
-  if (compl_pending != 0 && !got_int && !(cot_flags & kOptCotFlagNoinsert)
+  if (compl_pending && !got_int && !(cot_flags & kOptCotFlagNoinsert)
-      && !compl_autocomplete) {
+      && (!compl_autocomplete || ins_compl_has_preinsert())) {
     // Insert the first match immediately and advance compl_shown_match,
     // before finding other matches.
     int todo = compl_pending > 0 ? compl_pending : -compl_pending;
@@ -6599,3 +6599,11 @@ static void cpt_compl_refresh(void)
   // Make the list cyclic
   compl_matches = ins_compl_make_cyclic();
 }
+
+/// "preinserted()" function
+void f_preinserted(typval_T *argvars, typval_T *rettv, EvalFuncData fptr)
+{
+  if (ins_compl_preinsert_effect()) {
+    rettv->vval.v_number = 1;
+  }
+}

src/nvim/options.lua

@@ -1663,11 +1663,22 @@ local options = {
         	    to gather more alternatives for your candidate list,
         	    see 'completefuzzycollect'.
 
-           longest  Only insert the longest common text of the matches.  If
+           longest
-        	    the menu is displayed you can use CTRL-L to add more
+        	    When 'autocomplete' is not active, only the longest
-        	    characters.  Whether case is ignored depends on the kind
+        	    common prefix of the matches is inserted.  If the popup
-        	    of completion.  For buffer text the 'ignorecase' option is
+        	    menu is displayed, you can use CTRL-L to add more
-        	    used.
+        	    characters.  Whether case is ignored depends on the type
+        	    of completion.  For buffer text the 'ignorecase' option
+        	    applies.
+
+        	    When 'autocomplete' is active and no completion item is
+        	    selected, the longest common prefix of the matches is
+        	    inserted after the cursor.  The prefix is taken either
+        	    from all displayed items or only from items in the current
+        	    buffer.  The inserted text is highlighted with
+        	    |hl-PreInsert|, and the cursor position does not change
+        	    (similar to `"preinsert"`).  Press CTRL-Y to accept.
+        	    See also |preinserted()|.
 
            menu	    Use a popup menu to show the possible completions.  The
         	    menu is only shown when there is more than one match and
@@ -1700,22 +1711,21 @@ local options = {
         	    with "menu" or "menuone".  Overrides "preview".
 
            preinsert
-        	    When 'autocomplete' is not active, inserts the part of the
+        	    Inserts the text of the first completion candidate
-        	    first candidate word beyond the current completion leader,
+        	    beyond the current leader, highlighted with |hl-PreInsert|.
-        	    highlighted with |hl-PreInsert|.  The cursor doesn't move.
+        	    The cursor does not move.
-        	    Requires "fuzzy" unset and "menuone" in 'completeopt'.
+        	    Requires "fuzzy" to be unset, and either "menuone" in
-
+        	    'completeopt' or 'autocomplete' enabled.  When
-        	    When 'autocomplete' is active, inserts the longest common
+        	    'autocomplete' is enabled, this does not work if
-        	    prefix of matches (from all shown items or from the
+        	    'ignorecase' is set without 'infercase'.
-        	    current buffer items).  This occurs only when no menu item
+        	    See also |preinserted()|.
-        	    is selected.  Press CTRL-Y to accept.
 
            preview  Show extra information about the currently selected
         	    completion in the preview window.  Only works in
         	    combination with "menu" or "menuone".
 
-        Only "fuzzy", "popup", "preinsert" and "preview" have an effect when
+        Only "fuzzy", "longest", "popup", "preinsert" and "preview" have an
-        'autocomplete' is enabled.
+        effect when 'autocomplete' is enabled.
 
         This option does not apply to |cmdline-completion|.  See 'wildoptions'
         for that.

test/functional/editor/completion_spec.lua

@@ -1496,18 +1496,18 @@ describe('completion', function()
 
     -- During delay wait, user can open menu using CTRL_N completion
     feed('<Esc>')
-    command('set completeopt=menuone,preinsert')
+    command('set completeopt=menuone,longest')
     feed('Sf<C-N>')
     screen:expect([[
       foo                                                         |
       foobar                                                      |
       foobarbaz                                                   |
-      f{102:^oo}                                                         |
+      foo^                                                         |
-      {12:foo            }{1:                                             }|
+      {4:foo            }{1:                                             }|
       {4:foobar         }{1:                                             }|
       {4:foobarbaz      }{1:                                             }|
       {1:~                                                           }|*2
-      {5:-- Keyword completion (^N^P) }{6:match 1 of 3}                   |
+      {5:-- Keyword completion (^N^P) }{19:Back at original}               |
     ]])
 
     -- After the menu is open, ^N/^P and Up/Down should not delay

test/old/testdir/test_ins_complete.vim

@@ -124,10 +124,9 @@ endfunc
 func Test_omni_dash()
   func Omni(findstart, base)
     if a:findstart
-        return 5
+      return 5
     else
-        echom a:base
+      return ['-help', '-v']
-	return ['-help', '-v']
     endif
   endfunc
   set omnifunc=Omni
@@ -4231,6 +4230,145 @@ func Test_completeopt_preinsert()
   delfunc Omni_test
 endfunc
 
+func Test_autocomplete_completeopt_preinsert()
+  func Omni_test(findstart, base)
+    if a:findstart
+      return col(".") - 1
+    endif
+    return [#{word: "fobar"}, #{word: "foobar"}]
+  endfunc
+  set omnifunc=Omni_test complete+=o
+  set completeopt=preinsert autocomplete
+  " set completeopt=preinsert,menuone autocomplete
+  func GetLine()
+    let g:line = getline('.')
+    let g:col = col('.')
+  endfunc
+
+  call Ntest_override("char_avail", 1)
+  new
+  inoremap <buffer><F5> <C-R>=GetLine()<CR>
+  call feedkeys("Sfo\<F5>\<ESC>", 'tx')
+  call assert_equal("fobar", g:line)
+  call assert_equal(3, g:col)
+
+  call feedkeys("Sfoo\<F5>\<ESC>", 'tx')
+  call assert_equal("foobar", g:line)
+
+  call feedkeys("Sfoo\<BS>\<BS>\<BS>", 'tx')
+  call assert_equal("", getline('.'))
+
+  " delete a character
+  call feedkeys("Sfoo\<BS>b\<F5>\<ESC>", 'tx')
+  call assert_equal("fobar", g:line)
+  call assert_equal(4, g:col)
+
+  set complete&
+  %d
+  call setline(1, ['fobar', 'foobar'])
+
+  call feedkeys("Gofoo\<BS>\<BS>\<F5>\<ESC>", 'tx')
+  call assert_equal("fobar", g:line)
+  call assert_equal(2, g:col)
+
+  call feedkeys("Shello   wo\<Left>\<Left>\<Left>f\<F5>\<ESC>", 'tx')
+  call assert_equal("hello  fobar wo", g:line)
+  call assert_equal(9, g:col)
+
+  call feedkeys("Shello   wo\<Left>\<Left>\<Left>f\<BS>\<F5>\<ESC>", 'tx')
+  call assert_equal("hello   wo", g:line)
+  call assert_equal(8, g:col)
+
+  call feedkeys("Shello   wo\<Left>\<Left>\<Left>foo\<F5>\<ESC>", 'tx')
+  call assert_equal("hello  foobar wo", g:line)
+  call assert_equal(11, g:col)
+
+  call feedkeys("Shello   wo\<Left>\<Left>\<Left>foo\<BS>b\<F5>\<ESC>", 'tx')
+  call assert_equal("hello  fobar wo", g:line)
+  call assert_equal(11, g:col)
+
+  " confirm
+  call feedkeys("Sf\<C-Y>", 'tx')
+  call assert_equal("fobar", getline('.'))
+  call assert_equal(5, col('.'))
+
+  " cancel
+  call feedkeys("Sfo\<C-E>", 'tx')
+  call assert_equal("fo", getline('.'))
+  call assert_equal(2, col('.'))
+
+  " delete preinsert part
+  call feedkeys("Sfo ", 'tx')
+  call assert_equal("fo ", getline('.'))
+  call assert_equal(3, col('.'))
+
+  " can not work with fuzzy
+  set cot+=fuzzy
+  call feedkeys("Sf", 'tx')
+  call assert_equal("f", getline('.'))
+  set cot-=fuzzy
+
+  " does not work with 'ignorecase' unless 'infercase' is also enabled
+  %d
+  call setline(1, ['FIX', 'fobar', 'foobar'])
+  set ignorecase
+  call feedkeys("Gof\<F5>\<ESC>", 'tx')
+  call assert_equal("f", g:line)  " should not produce 'FIX'
+  set infercase
+  call feedkeys("Gof\<F5>\<ESC>", 'tx')
+  call assert_equal("fix", g:line)
+  set ignorecase& infercase&
+
+  %delete _
+  let &l:undolevels = &l:undolevels
+  normal! ifoo
+  let &l:undolevels = &l:undolevels
+  normal! obar
+  let &l:undolevels = &l:undolevels
+  normal! obaz
+  let &l:undolevels = &l:undolevels
+
+  func CheckUndo()
+    let g:errmsg = ''
+    call assert_equal(['foo', 'bar', 'baz'], getline(1, '$'))
+    undo
+    call assert_equal(['foo', 'bar'], getline(1, '$'))
+    undo
+    call assert_equal(['foo'], getline(1, '$'))
+    undo
+    call assert_equal([''], getline(1, '$'))
+    later 3
+    call assert_equal(['foo', 'bar', 'baz'], getline(1, '$'))
+    call assert_equal('', v:errmsg)
+  endfunc
+
+  " Check that switching buffer with "preinsert" doesn't corrupt undo.
+  new
+  setlocal bufhidden=wipe
+  inoremap <buffer> <F2> <Cmd>enew!<CR>
+  call feedkeys("if\<F2>\<Esc>", 'tx')
+  bwipe!
+  call CheckUndo()
+
+  " Check that closing window with "preinsert" doesn't corrupt undo.
+  new
+  setlocal bufhidden=wipe
+  inoremap <buffer> <F2> <Cmd>close!<CR>
+  call feedkeys("if\<F2>\<Esc>", 'tx')
+  call CheckUndo()
+
+  %delete _
+  delfunc CheckUndo
+
+  bw!
+  set cot&
+  set omnifunc&
+  set autocomplete&
+  call Ntest_override("char_avail", 0)
+  delfunc Omni_test
+  delfunc GetLine
+endfunc
+
 " Check that mark positions are correct after triggering multiline completion.
 func Test_complete_multiline_marks()
   func Omni_test(findstart, base)
@@ -5696,7 +5834,7 @@ func Test_autocompletedelay()
   call VerifyScreenDump(buf, 'Test_autocompletedelay_6', {})
 
   " During delay wait, user can open menu using CTRL_N completion
-  call term_sendkeys(buf, "\<Esc>:set completeopt=menuone,preinsert\<CR>")
+  call term_sendkeys(buf, "\<Esc>:set completeopt=menuone,longest\<CR>")
   call term_sendkeys(buf, "Sf\<C-N>")
   call VerifyScreenDump(buf, 'Test_autocompletedelay_7', {})
 
@@ -5720,16 +5858,18 @@ func Test_autocompletedelay()
   call StopVimInTerminal(buf)
 endfunc
 
-func Test_autocomplete_completeopt_preinsert()
+" Preinsert longest prefix when autocomplete
+func Test_autocomplete_longest()
   func GetLine()
     let g:line = getline('.')
     let g:col = col('.')
+    let g:preinserted = preinserted()
   endfunc
 
   call Ntest_override("char_avail", 1)
   new
   inoremap <buffer><F5> <C-R>=GetLine()<CR>
-  set completeopt=preinsert autocomplete
+  set completeopt=longest autocomplete
   call setline(1, ["foobar", "foozbar"])
   call feedkeys("Go\<ESC>", 'tx')
 
@@ -5761,10 +5901,15 @@ func Test_autocomplete_completeopt_preinsert()
   call DoTest("f\<C-P>", 'foobar', 7)
   call DoTest("fo\<BS>\<C-P>", 'foobar', 7)
 
+  " <C-Y> to accept preinserted text
+  inoremap <buffer><F6> <C-R>=pumvisible()<CR>
   call DoTest("zar\<C-O>0f", 'foozar', 2)
   call DoTest("zar\<C-O>0f\<C-Y>", 'foozar', 4)
+  call DoTest("zar\<C-O>0f\<C-Y>\<F6>", 'foo1zar', 5)
   call DoTest("zar\<C-O>0f\<C-Y>\<BS>", 'foozar', 3)
+  call DoTest("zar\<C-O>0f\<C-Y>\<BS>\<F6>", 'fo1zar', 4)
 
+  " Select items in menu
   call DoTest("zar\<C-O>0f\<C-N>", 'foozbarzar', 8)
   call DoTest("zar\<C-O>0f\<C-N>\<C-N>", 'foobarzar', 7)
   call DoTest("zar\<C-O>0f\<C-N>\<C-N>\<C-N>", 'foozar', 2)
@@ -5778,6 +5923,34 @@ func Test_autocomplete_completeopt_preinsert()
   call DoTest("f", 'f', 2)
   set cot-=fuzzy
 
+  " preinserted()
+  call DoTest("f", 'foo', 2)
+  call assert_equal(1, g:preinserted)
+  call assert_equal(0, preinserted())
+  call DoTest("f\<BS>", '', 1)
+  call assert_equal(0, g:preinserted)
+  call DoTest("f ", 'f ', 3)
+  call assert_equal(0, g:preinserted)
+  call DoTest("foob", 'foobar', 5)
+  call assert_equal(1, g:preinserted)
+  call DoTest("foob\<BS>\<BS>x", 'fox', 4)
+  call assert_equal(0, g:preinserted)
+
+  " Complete non-keyword
+  func Omni(findstart, base)
+    if a:findstart
+        return 5
+    else
+	return ['xyz:']
+    endif
+  endfunc
+  set omnifunc=Omni
+  set cpt+=o
+  call DoTest("xyz:", "xyz:xyz:", 5)
+  call DoTest("xyz:\<BS>\<BS>", "xyz:", 3)
+  set omnifunc& cpt&
+  delfunc Omni
+
   " leader should match prefix of inserted word
   %delete
   set smartcase ignorecase
@@ -5788,6 +5961,16 @@ func Test_autocomplete_completeopt_preinsert()
   call assert_equal('FOO', g:line)
   set smartcase& ignorecase&
 
+  " avoid repeating text that is already present after the cursor
+  %delete
+  call setline(1, ["foobarbaz", ""])
+  call feedkeys($"G", 'tx')
+  call DoTest("baz\<C-O>0f", "foobarbaz", 2)
+  call feedkeys($"Sxfoozar\<CR>\<Esc>", 'tx')
+  call DoTest("baz\<C-O>0f", "foobarbaz", 2)
+  call feedkeys($"Sfoozar\<CR>\<Esc>", 'tx')
+  call DoTest("baz\<C-O>0f", "foobaz", 2)
+
   " Verify that redo (dot) works
   %delete
   call setline(1, ["foobar", "foozbar", "foobaz", "changed", "change"])
@@ -5824,7 +6007,7 @@ func Test_autocomplete_completeopt_preinsert()
     call assert_equal('', v:errmsg)
   endfunc
 
-  " Check that switching buffer with "preinsert" doesn't corrupt undo.
+  " Check that switching buffer with "longest" doesn't corrupt undo.
   new
   setlocal bufhidden=wipe
   inoremap <buffer> <F2> <Cmd>enew!<CR>
@@ -5832,7 +6015,7 @@ func Test_autocomplete_completeopt_preinsert()
   bwipe!
   call CheckUndo()
 
-  " Check that closing window with "preinsert" doesn't corrupt undo.
+  " Check that closing window with "longest" doesn't corrupt undo.
   new
   setlocal bufhidden=wipe
   inoremap <buffer> <F2> <Cmd>close!<CR>