patch 8.0.0102
Problem:    Cannot set 'dictionary' to a path.
Solution:   Allow for slash and backslash.  Add a test (partly by Daisuke
            Suzuki, closes #1279, closes #1284)
diff --git a/src/option.c b/src/option.c
index 52330f7..5b6dbe3 100644
--- a/src/option.c
+++ b/src/option.c
@@ -452,10 +452,11 @@
 #define P_NFNAME       0x400000L /* only normal file name chars allowed */
 #define P_INSECURE     0x800000L /* option was set from a modeline */
 #define P_PRI_MKRC    0x1000000L /* priority for :mkvimrc (setting option has
-				   side effects) */
+				    side effects) */
 #define P_NO_ML       0x2000000L /* not allowed in modeline */
 #define P_CURSWANT    0x4000000L /* update curswant required; not needed when
 				  * there is a redraw flag */
+#define P_NDNAME      0x8000000L /* only normal dir name chars allowed */
 
 #define ISK_LATIN1  (char_u *)"@,48-57,_,192-255"
 
@@ -992,7 +993,7 @@
 			    (char_u *)NULL, PV_NONE,
 #endif
 			    {(char_u *)FALSE, (char_u *)0L} SCRIPTID_INIT},
-    {"dictionary",  "dict", P_STRING|P_EXPAND|P_VI_DEF|P_ONECOMMA|P_NODUP|P_NFNAME,
+    {"dictionary",  "dict", P_STRING|P_EXPAND|P_VI_DEF|P_ONECOMMA|P_NODUP|P_NDNAME,
 #ifdef FEAT_INS_EXPAND
 			    (char_u *)&p_dict, PV_DICT,
 #else
@@ -5876,11 +5877,13 @@
 	errmsg = e_secure;
     }
 
-    /* Check for a "normal" file name in some options.  Disallow a path
-     * separator (slash and/or backslash), wildcards and characters that are
-     * often illegal in a file name. */
-    else if ((options[opt_idx].flags & P_NFNAME)
+    /* Check for a "normal" directory or file name in some options.  Disallow a
+     * path separator (slash and/or backslash), wildcards and characters that
+     * are often illegal in a file name. */
+    else if (((options[opt_idx].flags & P_NFNAME)
 		    && vim_strpbrk(*varp, (char_u *)"/\\*?[|;&<>\r\n") != NULL)
+	  || ((options[opt_idx].flags & P_NDNAME)
+		    && vim_strpbrk(*varp, (char_u *)"*?[|;&<>\r\n") != NULL))
     {
 	errmsg = e_invarg;
     }