vim-patch:8.2.1401: cannot jump to the last used tabpage

Problem:    Cannot jump to the last used tabpage.
Solution:   Add g<Tab> and tabpagnr('#'). (Yegappan Lakshmanan, closes vim/vim#6661,
            neovim #11626)
https://github.com/vim/vim/commit/62a232506d06f6d1b3b7271801c907d6294dfe84

Nvim implemented this feature before Vim, but Vim made some useful changes (e.g:
beeping on failure). Port the changes to closer match Vim (also makes porting
future patches easier).

Also note that because CHECK_CMDWIN was added to goto_tabpage_tp, there is no
need to do the extra work with tabpage_index and goto_tabpage inside
goto_tabpage_lastused to fix cmdwin issues any more (#11692).
Note that while goto_tabpage_tp doesn't check for textlock like goto_tabpage
does, it shouldn't matter as it is already checked for earlier.

Add tags for <C-Tab> to tabpage.txt, and refer to <C-Tab> over CTRL-Tab to be
consistent with other docs like the patch.
Remove mention of "previous tabpage" (it can be confused with the tabpage to the
left, e.g: `:tabprevious`).
Similarly, don't rename old_curtab to last_tab in enter_tabpage (it might be
confused with the right-most tabpage, e.g: `:tablast`).

Cherry-pick Test_tabpage change from v8.2.0634.
https://github.com/vim/vim/commit/92b83ccfda7a1d654ccaaf161a9c8a8e01fbcf76

8 files changed, 80 insertions, 28 deletions

diff --git a/runtime/doc/builtin.txt b/runtime/doc/builtin.txt
index ccc02e83b2..9eec23b7b7 100644
--- a/runtime/doc/builtin.txt
+++ b/runtime/doc/builtin.txt
@@ -8103,15 +8103,15 @@ tabpagebuflist([{arg}])					*tabpagebuflist()*
 tabpagenr([{arg}])					*tabpagenr()*
 		The result is a Number, which is the number of the current
 		tab page.  The first tab page has number 1.
+
 		The optional argument {arg} supports the following values:
 			$	the number of the last tab page (the tab page
 				count).
-			#	the number of the last accessed tab page (where
-				|g<Tab>| goes to).  If there is no previous
-				tab page, 0 is returned.
+			#	the number of the last accessed tab page
+				(where |g<Tab>| goes to).  If there is no
+				previous tab page, 0 is returned.
 		The number can be used with the |:tab| command.
 
-
 tabpagewinnr({tabarg} [, {arg}])			*tabpagewinnr()*
 		Like |winnr()| but for tab page {tabarg}.
 		{tabarg} specifies the number of tab page to be used.
diff --git a/runtime/doc/index.txt b/runtime/doc/index.txt
index d02ab1b759..572b4e3f93 100644
--- a/runtime/doc/index.txt
+++ b/runtime/doc/index.txt
@@ -431,6 +431,7 @@ tag		char	      note action in Normal mode	~
 |<C-LeftMouse>|	<C-LeftMouse>	   ":ta" to the keyword at the mouse click
 |<C-Right>|	<C-Right>	1  same as "w"
 |<C-RightMouse>| <C-RightMouse>	   same as "CTRL-T"
+|<C-Tab>|	<C-Tab>		   same as "g<Tab>"
 |<Del>|		["x]<Del>	2  same as "x"
 |N<Del>|	{count}<Del>	   remove the last digit from {count}
 |<Down>|	<Down>		1  same as "j"
@@ -577,7 +578,7 @@ tag		command		   action in Normal mode	~
 				   following the file name.
 |CTRL-W_gt|	CTRL-W g t	   same as `gt`: go to next tab page
 |CTRL-W_gT|	CTRL-W g T	   same as `gT`: go to previous tab page
-|CTRL-W_g<Tab>|	CTRL-W g <Tab>	   same as `g<Tab>` : go to last accessed tab
+|CTRL-W_g<Tab>|	CTRL-W g <Tab>	   same as |g<Tab>|: go to last accessed tab
 				   page
 |CTRL-W_h|	CTRL-W h	   go to Nth left window (stop at first window)
 |CTRL-W_i|	CTRL-W i	   split window and jump to declaration of
diff --git a/runtime/doc/tabpage.txt b/runtime/doc/tabpage.txt
index 78b5101da7..f06a6bcc34 100644
--- a/runtime/doc/tabpage.txt
+++ b/runtime/doc/tabpage.txt
@@ -196,10 +196,6 @@ gt					*i_CTRL-<PageDown>* *i_<C-PageDown>*
 {count}<C-PageDown>
 {count}gt	Go to tab page {count}.  The first tab page has number one.
 
-CTRL-<Tab>					*CTRL-<Tab>*
-CTRL-W g<Tab>					*g<Tab>* *CTRL-W_g<Tab>*
-g<Tab>		Go to previous (last accessed) tab page.
-
 :tabp[revious]				*:tabp* *:tabprevious* *gT* *:tabN*
 :tabN[ext]				*:tabNext* *CTRL-<PageUp>*
 <C-PageUp>			 *<C-PageUp>* *i_CTRL-<PageUp>* *i_<C-PageUp>*
@@ -219,6 +215,9 @@ gT		Go to the previous tab page.  Wraps around from the first one
 							*:tabl* *:tablast*
 :tabl[ast]	Go to the last tab page.
 
+<C-Tab>						*CTRL-<Tab>* *<C-Tab>*
+CTRL-W g<Tab>					*g<Tab>* *CTRL-W_g<Tab>*
+g<Tab>		Go to the last accessed tab page.
 
 Other commands:
 							*:tabs*
diff --git a/src/nvim/eval/funcs.c b/src/nvim/eval/funcs.c
index b249dffe11..b74f9759ac 100644
--- a/src/nvim/eval/funcs.c
+++ b/src/nvim/eval/funcs.c
@@ -11213,9 +11213,7 @@ static void f_tabpagenr(typval_T *argvars, typval_T *rettv, FunPtr fptr)
       if (strcmp(arg, "$") == 0) {
         nr = tabpage_index(NULL) - 1;
       } else if (strcmp(arg, "#") == 0) {
-        nr = valid_tabpage(lastused_tabpage)
-             ? tabpage_index(lastused_tabpage)
-             : nr;
+        nr = valid_tabpage(lastused_tabpage) ? tabpage_index(lastused_tabpage) : 0;
       } else {
         semsg(_(e_invexpr2), arg);
       }
diff --git a/src/nvim/globals.h b/src/nvim/globals.h
index b64ed7c758..cbd67afb09 100644
--- a/src/nvim/globals.h
+++ b/src/nvim/globals.h
@@ -448,10 +448,11 @@ EXTERN int aucmd_win_used INIT(= false);  // aucmd_win is being used
 EXTERN frame_T *topframe;      // top of the window frame tree
 
 // Tab pages are alternative topframes.  "first_tabpage" points to the first
-// one in the list, "curtab" is the current one.
+// one in the list, "curtab" is the current one. "lastused_tabpage" is the
+// last used one.
 EXTERN tabpage_T *first_tabpage;
-EXTERN tabpage_T *lastused_tabpage;
 EXTERN tabpage_T *curtab;
+EXTERN tabpage_T *lastused_tabpage;
 EXTERN bool redraw_tabline INIT(= false);  // need to redraw tabline
 
 // Iterates over all tabs in the tab list
diff --git a/src/nvim/normal.c b/src/nvim/normal.c
index 0e5e0ab403..f402865d2d 100644
--- a/src/nvim/normal.c
+++ b/src/nvim/normal.c
@@ -5869,7 +5869,7 @@ static void nv_gomark(cmdarg_T *cap)
   }
 }
 
-// Handle CTRL-O, CTRL-I, "g;", "g,", and "CTRL-Tab" commands.
+/// Handle CTRL-O, CTRL-I, "g;", "g,", and "CTRL-Tab" commands.
 static void nv_pcmark(cmdarg_T *cap)
 {
   pos_T *pos;
@@ -5878,7 +5878,9 @@ static void nv_pcmark(cmdarg_T *cap)
 
   if (!checkclearopq(cap->oap)) {
     if (cap->cmdchar == TAB && mod_mask == MOD_MASK_CTRL) {
-      goto_tabpage_lastused();
+      if (!goto_tabpage_lastused()) {
+        clearopbeep(cap->oap);
+      }
       return;
     }
     if (cap->cmdchar == 'g') {
@@ -6642,9 +6644,10 @@ static void nv_g_cmd(cmdarg_T *cap)
       goto_tabpage(-(int)cap->count1);
     }
     break;
+
   case TAB:
-    if (!checkclearop(oap)) {
-      goto_tabpage_lastused();
+    if (!checkclearop(oap) && !goto_tabpage_lastused()) {
+      clearopbeep(oap);
     }
     break;
 
diff --git a/src/nvim/testdir/test_tabpage.vim b/src/nvim/testdir/test_tabpage.vim
index 9869dc7590..b458f9a311 100644
--- a/src/nvim/testdir/test_tabpage.vim
+++ b/src/nvim/testdir/test_tabpage.vim
@@ -128,6 +128,8 @@ function Test_tabpage()
   1tabmove
   call assert_equal(2, tabpagenr())
 
+  call assert_fails('let t = tabpagenr("@")', 'E15:')
+  call assert_equal(0, tabpagewinnr(-1))
   call assert_fails("99tabmove", 'E16:')
   call assert_fails("+99tabmove", 'E16:')
   call assert_fails("-99tabmove", 'E16:')
@@ -683,4 +685,48 @@ func Test_tabline_tabmenu()
   %bw!
 endfunc
 
+" Test for jumping to last accessed tabpage
+func Test_lastused_tabpage()
+  tabonly!
+  call assert_equal(0, tabpagenr('#'))
+  call assert_beeps('call feedkeys("g\<Tab>", "xt")')
+  call assert_beeps('call feedkeys("\<C-Tab>", "xt")')
+  call assert_beeps('call feedkeys("\<C-W>g\<Tab>", "xt")')
+
+  " open four tab pages
+  tabnew
+  tabnew
+  tabnew
+
+  2tabnext
+
+  " Test for g<Tab>
+  call assert_equal(4, tabpagenr('#'))
+  call feedkeys("g\<Tab>", "xt")
+  call assert_equal(4, tabpagenr())
+  call assert_equal(2, tabpagenr('#'))
+
+  " Test for <C-Tab>
+  call feedkeys("\<C-Tab>", "xt")
+  call assert_equal(2, tabpagenr())
+  call assert_equal(4, tabpagenr('#'))
+
+  " Test for <C-W>g<Tab>
+  call feedkeys("\<C-W>g\<Tab>", "xt")
+  call assert_equal(4, tabpagenr())
+  call assert_equal(2, tabpagenr('#'))
+
+  " Try to jump to a closed tab page
+  tabclose 2
+  call assert_equal(0, tabpagenr('#'))
+  call feedkeys("g\<Tab>", "xt")
+  call assert_equal(3, tabpagenr())
+  call feedkeys("\<C-Tab>", "xt")
+  call assert_equal(3, tabpagenr())
+  call feedkeys("\<C-W>g\<Tab>", "xt")
+  call assert_equal(3, tabpagenr())
+
+  tabclose!
+endfunc
+
 " vim: shiftwidth=2 sts=2 expandtab
diff --git a/src/nvim/window.c b/src/nvim/window.c
index f2b84a4124..a235b07b47 100644
--- a/src/nvim/window.c
+++ b/src/nvim/window.c
@@ -530,10 +530,6 @@ wingotofile:
       do_nv_ident('g', xchar);
       break;
 
-    case TAB:
-      goto_tabpage_lastused();
-      break;
-
     case 'f':                       // CTRL-W gf: "gf" in a new tab page
     case 'F':                       // CTRL-W gF: "gF" in a new tab page
       cmdmod.tab = tabpage_index(curtab) + 1;
@@ -547,6 +543,12 @@ wingotofile:
       goto_tabpage(-(int)Prenum1);
       break;
 
+    case TAB:                       // CTRL-W g<Tab>: go to last used tab page
+      if (!goto_tabpage_lastused()) {
+        beep_flush();
+      }
+      break;
+
     case 'e':
       if (curwin->w_floating || !ui_has(kUIMultigrid)) {
         beep_flush();
@@ -4119,8 +4121,8 @@ static void enter_tabpage(tabpage_T *tp, buf_T *old_curbuf, bool trigger_enter_a
 {
   int old_off = tp->tp_firstwin->w_winrow;
   win_T *next_prevwin = tp->tp_prevwin;
-
   tabpage_T *old_curtab = curtab;
+
   curtab = tp;
   firstwin = tp->tp_firstwin;
   lastwin = tp->tp_lastwin;
@@ -4291,13 +4293,15 @@ void goto_tabpage_tp(tabpage_T *tp, bool trigger_enter_autocmds, bool trigger_le
   }
 }
 
-// Go to the last accessed tab page, if there is one.
-void goto_tabpage_lastused(void)
+/// Go to the last accessed tab page, if there is one.
+/// @return true if the tab page is valid, false otherwise.
+bool goto_tabpage_lastused(void)
 {
-  int index = tabpage_index(lastused_tabpage);
-  if (index < tabpage_index(NULL)) {
-    goto_tabpage(index);
+  if (valid_tabpage(lastused_tabpage)) {
+    goto_tabpage_tp(lastused_tabpage, true, true);
+    return true;
   }
+  return false;
 }
 
 /*