patch 8.2.4498: using <Plug> with "noremap" does not work

Problem:    Using <Plug> with "noremap" does not work.
Solution:   Always remap <Plug>. (closes #9879, closes #9789)

runtime/doc/map.txt

@@ -80,6 +80,8 @@ modes.
 			where the map command applies.  Disallow mapping of
 			{rhs}, to avoid nested and recursive mappings.  Often
 			used to redefine a command.
+			Note: When <Plug> appears in the {rhs} this part is
+			always applied even if remapping is disallowed.
 
 
 :unm[ap]  {lhs}			|mapmode-nvo|		*:unm*  *:unmap*

src/getchar.c

@@ -2424,6 +2424,13 @@ handle_mapping(
     int		keylen = *keylenp;
     int		i;
     int		local_State = get_real_state();
+    int		is_plug_map = FALSE;
+
+    // If typehead starts with <Plug> then remap, even for a "noremap" mapping.
+    if (typebuf.tb_buf[typebuf.tb_off] == K_SPECIAL
+	    && typebuf.tb_buf[typebuf.tb_off + 1] == KS_EXTRA
+	    && typebuf.tb_buf[typebuf.tb_off + 2] == KE_PLUG)
+	is_plug_map = TRUE;
 
     /*
      * Check for a mappable key sequence.
@@ -2441,7 +2448,7 @@ handle_mapping(
     tb_c1 = typebuf.tb_buf[typebuf.tb_off];
     if (no_mapping == 0 && is_maphash_valid()
 	    && (no_zero_mapping == 0 || tb_c1 != '0')
-	    && (typebuf.tb_maplen == 0
+	    && (typebuf.tb_maplen == 0 || is_plug_map
 		|| (p_remap
 		    && (typebuf.tb_noremap[typebuf.tb_off]
 				    & (RM_NONE|RM_ABBR)) == 0))
@@ -2562,7 +2569,7 @@ handle_mapping(
 		    for (n = mlen; --n >= 0; )
 			if (*s++ & (RM_NONE|RM_ABBR))
 			    break;
-		    if (n >= 0)
+		    if (!is_plug_map && n >= 0)
 			continue;
 
 		    if (keylen > typebuf.tb_len)

src/testdir/test_mapping.vim

@@ -1536,4 +1536,34 @@ func Test_abbreviate_latin1_encoding()
   set encoding=utf-8
 endfunc
 
+" Test for <Plug> always being mapped, even when used with "noremap".
+func Test_plug_remap()
+  let g:foo = 0
+  nnoremap <Plug>(Increase_x) <Cmd>let g:foo += 1<CR>
+  nmap <F2> <Plug>(Increase_x)
+  nnoremap <F3> <Plug>(Increase_x)
+  call feedkeys("\<F2>", 'xt')
+  call assert_equal(1, g:foo)
+  call feedkeys("\<F3>", 'xt')
+  call assert_equal(2, g:foo)
+  nnoremap x <Nop>
+  nmap <F4> x<Plug>(Increase_x)x
+  nnoremap <F5> x<Plug>(Increase_x)x
+  call setline(1, 'Some text')
+  normal! gg$
+  call feedkeys("\<F4>", 'xt')
+  call assert_equal(3, g:foo)
+  call assert_equal('Some text', getline(1))
+  call feedkeys("\<F5>", 'xt')
+  call assert_equal(4, g:foo)
+  call assert_equal('Some te', getline(1))
+  nunmap <Plug>(Increase_x)
+  nunmap <F2>
+  nunmap <F3>
+  nunmap <F4>
+  nunmap <F5>
+  unlet g:foo
+  %bw!
+endfunc
+
 " vim: shiftwidth=2 sts=2 expandtab

src/version.c

@@ -754,6 +754,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    4498,
 /**/
     4497,
 /**/