vim-patch:8.1.2348: :const cannot be followed by "| endif"

Problem:    :const cannot be followed by "| endif".
Solution:   Check following command for :const. (closes vim/vim#5269)
            Also fix completion after :const.
8f76e6b12b

src/nvim/eval.c

@@ -2872,7 +2872,7 @@ void set_context_for_expression(expand_T *xp, char_u *arg, cmdidx_T cmdidx)
   int c;
   char_u      *p;
 
-  if (cmdidx == CMD_let) {
+  if (cmdidx == CMD_let || cmdidx == CMD_const) {
     xp->xp_context = EXPAND_USER_VARS;
     if (vim_strpbrk(arg, (char_u *)"\"'+-*/%.=!?~|&$([<>,#") == NULL) {
       /* ":let var1 var2 ...": find last space. */

src/nvim/ex_docmd.c

@@ -2126,6 +2126,7 @@ static char_u * do_one_cmd(char_u **cmdlinep,
     case CMD_browse:
     case CMD_call:
     case CMD_confirm:
+    case CMD_const:
     case CMD_delfunction:
     case CMD_djump:
     case CMD_dlist:
@@ -3437,6 +3438,7 @@ const char * set_one_cmd_context(
   case CMD_syntax:
     set_context_in_syntax_cmd(xp, arg);
     break;
+  case CMD_const:
   case CMD_let:
   case CMD_if:
   case CMD_elseif:

src/nvim/testdir/test_cmdline.vim

@@ -159,6 +159,7 @@ func Test_expr_completion()
   endif
   for cmd in [
 	\ 'let a = ',
+	\ 'const a = ',
 	\ 'if',
 	\ 'elseif',
 	\ 'while',

src/nvim/testdir/test_const.vim

@@ -176,6 +176,12 @@ func Test_cannot_modify_existing_variable()
     call assert_fails('const [i2, f2, s2] = [1, 1.1, "vim"]', 'E995:')
 endfunc
 
+func Test_const_with_condition()
+  const x = 0
+  if 0 | const x = 1 | endif
+  call assert_equal(0, x)
+endfunc
+
 func Test_const_with_index_access()
     let l = [1, 2, 3]
     call assert_fails('const l[0] = 4', 'E996:')

src/nvim/testdir/test_let.vim

@@ -24,6 +24,10 @@ func Test_let()
   let out = execute('let a {0 == 1 ? "a" : "b"}')
   let s = "\na                     #1\nb                     #2"
   call assert_equal(s, out)
+
+  let x = 0
+  if 0 | let x = 1 | endif
+  call assert_equal(0, x)
 endfunc
 
 func s:set_arg1(a) abort