patch 8.2.1657: Vim9: no proper error for nested ":def!"

Problem:    Vim9: no proper error for nested ":def!".
Solution:   Check for "!". (closes #6920)

src/errors.h

@@ -256,4 +256,6 @@ EXTERN char e_assert_fails_fourth_argument[]
 	INIT(= N_("E1115: assert_fails() fourth argument must be a number"));
 EXTERN char e_assert_fails_fifth_argument[]
 	INIT(= N_("E1116: assert_fails() fifth argument must be a string"));
+EXTERN char e_cannot_use_bang_with_nested_def[]
+	INIT(= N_("E1117: Cannot use ! with nested :def"));
 #endif

src/testdir/test_vim9_func.vim

@@ -134,6 +134,23 @@ def Test_nested_function()
   CheckDefFailure(['func Nested()', 'endfunc'], 'E1086:')
   CheckDefFailure(['def s:Nested()', 'enddef'], 'E1075:')
   CheckDefFailure(['def b:Nested()', 'enddef'], 'E1075:')
+
+  CheckDefFailure([
+        'def Outer()',
+        '  def Inner()',
+        '    # comment',
+        '  enddef',
+        '  def Inner()',
+        '  enddef',
+        'enddef'], 'E1073:')
+  CheckDefFailure([
+        'def Outer()',
+        '  def Inner()',
+        '    # comment',
+        '  enddef',
+        '  def! Inner()',
+        '  enddef',
+        'enddef'], 'E1117:')
 enddef
 
 func Test_call_default_args_from_func()

src/version.c

@@ -750,6 +750,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1657,
 /**/
     1656,
 /**/

src/vim9compile.c

@@ -4320,6 +4320,12 @@ compile_nested_function(exarg_T *eap, cctx_T *cctx)
     ufunc_T	*ufunc;
     int		r;
 
+    if (*name_start == '!')
+    {
+	emsg(_(e_cannot_use_bang_with_nested_def));
+	return NULL;
+    }
+
     // Only g:Func() can use a namespace.
     if (name_start[1] == ':' && !is_global)
     {