add missing two-weapon feedback

When dual-wielding and you wield a different weapon, two weapon combat
was silently toggled off even when the new weapon was eligible to be
dual-wielded.  If the verbose flag is On, explicitly tell the player
when wielding something toggles off two-weapon mode.  Wielding '-' is
an exception because you already get told that you're empty handed.

doc/fixes37.0

@@ -117,6 +117,7 @@ demon lords hate Demonbane
 pets avoid eating shapeshifter corpses unless starving or nearly feral
 blessed scroll of teleportation gives a single controlled teleport
 allow opening a tin without interruption if slimed
+tell player when wielding a different weapon toggles off dual-wielding
 
 
 Fixes to 3.7.0-x Problems that Were Exposed Via git Repository

src/wield.c

@@ -65,6 +65,19 @@ static int FDECL(ready_weapon, (struct obj *));
 #define will_weld(optr) \
     ((optr)->cursed && (erodeable_wep(optr) || (optr)->otyp == TIN_OPENER))
 
+/* to dual-wield, 'obj' must be a weapon or a weapon-tool, and not a bow
+   or arrow or missile (dart, shuriken, boomerang), so not matching the
+   one-handed weapons which yield "you begin bashing" if used for melee;
+   empty hands and two-handed weapons have to be handled separately */
+#define TWOWEAPOK(obj) \
+    (((obj)->oclass == WEAPON_CLASS)                            \
+     ? !(is_launcher(obj) ||is_ammo(obj) || is_missile(obj))    \
+     : is_weptool(obj))
+
+static const char
+    are_no_longer_twoweap[] = "are no longer using two weapons at once",
+    can_no_longer_twoweap[] = "can no longer wield two weapons at once";
+
 /*** Functions that place a given item in a slot ***/
 /* Proper usage includes:
  * 1.  Initializing the slot during character generation or a
@@ -139,7 +152,7 @@ struct obj *wep;
 {
     /* Separated function so swapping works easily */
     int res = 0;
-    boolean had_wep = (uwep != 0);
+    boolean was_twoweap = u.twoweap, had_wep = (uwep != 0);
 
     if (!wep) {
         /* No weapon */
@@ -192,7 +205,16 @@ struct obj *wep;
             prinv((char *) 0, wep, 0L);
             wep->owornmask = dummy;
         }
+
         setuwep(wep);
+        if (was_twoweap && !u.twoweap && flags.verbose) {
+            /* skip this message if we already got "empty handed" one above;
+               also, Null is not safe for neither TWOWEAPOK() or bimanual() */
+            if (uwep)
+                You("%s.", ((TWOWEAPOK(uwep) && !bimanual(uwep))
+                            ? are_no_longer_twoweap
+                            : can_no_longer_twoweap));
+        }
 
         /* KMH -- Talking artifacts are finally implemented */
         arti_speak(wep);
@@ -571,7 +593,7 @@ dowieldquiver()
         You("are now empty %s.", body_part(HANDED));
         res = 1;
     } else if (was_twoweap && !u.twoweap) {
-        You("are no longer wielding two weapons at once.");
+        You("%s.", are_no_longer_twoweap);
         res = 1;
     }
     return res;
@@ -662,16 +684,6 @@ can_twoweapon()
 {
     struct obj *otmp;
 
-    /* to dual-wield, obj must be a weapon or a weapon-tool, and not
-       a bow or arrow or missile (dart, shuriken, boomerang), matching
-       the sorts of weapons which yield "you begin bashing" when used
-       for melee; we don't bother including polearms here because
-       they'll be rejected as two-weapon because they're two-handed */
-#define TWOWEAPOK(obj) \
-    (((obj)->oclass == WEAPON_CLASS)                            \
-     ? !(is_launcher(obj) ||is_ammo(obj) || is_missile(obj))    \
-     : is_weptool(obj))
-
     if (!could_twoweap(g.youmonst.data)) {
         if (Upolyd)
             You_cant("use two weapons in your current form.");
@@ -797,7 +809,7 @@ void
 untwoweapon()
 {
     if (u.twoweap) {
-        You("can no longer use two weapons at once.");
+        You("%s.", can_no_longer_twoweap);
         set_twoweap(FALSE); /* u.twoweap = FALSE */
         update_inventory();
     }