patch 8.1.2187: error for bad regexp even though regexp is not used

Problem:    Error for bad regexp even though regexp is not used when writing
            a file. (Arseny Nasokin)
Solution:   Ignore regexp errors. (closes #5059)

src/cmdexpand.c

@@ -1976,6 +1976,8 @@ ExpandFromContext(
 	flags |= EW_KEEPALL;
     if (options & WILD_SILENT)
 	flags |= EW_SILENT;
+    if (options & WILD_NOERROR)
+	flags |= EW_NOERROR;
     if (options & WILD_ALLLINKS)
 	flags |= EW_ALLLINKS;
 

src/ex_docmd.c

@@ -4211,7 +4211,8 @@ expand_filename(
 		else /* n == 2 */
 		{
 		    expand_T	xpc;
-		    int		options = WILD_LIST_NOTFOUND|WILD_ADD_SLASH;
+		    int		options = WILD_LIST_NOTFOUND
+					       | WILD_NOERROR | WILD_ADD_SLASH;
 
 		    ExpandInit(&xpc);
 		    xpc.xp_context = EXPAND_FILES;

src/testdir/test_writefile.vim

@@ -1,4 +1,4 @@
-" Tests for the writefile() function.
+" Tests for the writefile() function and some :write commands.
 
 func Test_writefile()
   let f = tempname()
@@ -16,6 +16,11 @@ func Test_writefile()
   call delete(f)
 endfunc
 
+func Test_writefile_ignore_regexp_error()
+  write Xt[z-a]est.txt
+  call delete('Xt[z-a]est.txt')
+endfunc
+
 func Test_writefile_fails_gently()
   call assert_fails('call writefile(["test"], "Xfile", [])', 'E730:')
   call assert_false(filereadable("Xfile"))

src/version.c

@@ -741,6 +741,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    2187,
 /**/
     2186,
 /**/