feedback when seeing hider-under hide under

If an monster hides under and object or under water while in view,
say so.

Also, the sanity check for a hidden eel wasn't consistent with the
hiding criteria for it.  An eel can't hide on the Plane of Water even
when it's in the water rather than in an air bubble.

doc/fixes3-7-0.txt

@@ -1193,6 +1193,7 @@ skip sanity_check handling when current command is ^P, otherwise it might
 applying a cream pie (always) or lump of royal jelly (sometimes) would use up
 	the object and then access its memory after that had been freed
 keep track of hero's pending movement points across save and restore
+give feedback if hero sees a monster become hidden under an object or water
 
 
 Fixes to 3.7.0-x General Problems Exposed Via git Repository

src/mon.c

@@ -152,15 +152,19 @@ sanity_check_single_mon(
         if (m_at(mx, my) == mtmp && hides_under(mptr) && !OBJ_AT(mx, my))
             impossible("mon hiding under nonexistent obj (%s)", msg);
         if (mptr->mlet == S_EEL
-            && !is_pool(mx, my) && !Is_waterlevel(&u.uz))
+            && !(is_pool(mx, my) && !Is_waterlevel(&u.uz)))
-            impossible("eel hiding out of water (%s)", msg);
+            impossible("eel hiding %s (%s)",
+                       !Is_waterlevel(&u.uz) ? "out of water"
+                                             : "on Plane of Water", msg);
         if (ceiling_hider(mptr)
             /* normally !accessible would be overridable with passes_walls,
                but not for hiding on the ceiling */
             && (!has_ceiling(&u.uz) ||
                 !(levl[mx][my].typ == POOL
                   || levl[mx][my].typ == MOAT
+                  || levl[mx][my].typ == WATER
                   || levl[mx][my].typ == LAVAPOOL
+                  || levl[mx][my].typ == LAVAWALL
                   || accessible(mx, my))))
             impossible("ceiling hider hiding %s (%s)",
                        !has_ceiling(&u.uz) ? "without ceiling"
@@ -4189,6 +4193,8 @@ boolean
 hideunder(struct monst *mtmp)
 {
     struct trap *t;
+    const char *seenmon = (char *) 0, *seenobj = (char *) 0;
+    int seeit = canseemon(mtmp);
     boolean oldundetctd, undetected = FALSE, is_u = (mtmp == &gy.youmonst);
     coordxy x = is_u ? u.ux : mtmp->mx, y = is_u ? u.uy : mtmp->my;
 
@@ -4200,16 +4206,20 @@ hideunder(struct monst *mtmp)
         ; /* undetected==FALSE; can't hide while stuck in a non-pit trap */
     } else if (mtmp->data->mlet == S_EEL) {
         undetected = (is_pool(x, y) && !Is_waterlevel(&u.uz));
+        if (seeit)
+            seenobj = "the water";
     } else if (hides_under(mtmp->data) && OBJ_AT(x, y)) {
         struct obj *otmp = gl.level.objects[x][y];
 
+        if (seeit)
+            seenobj = ansimpleoname(otmp);
         /* most monsters won't hide under cockatrice corpse but they
            can hide under a pile containing more than just such corpses */
         while (otmp && otmp->otyp == CORPSE
                && touch_petrifies(&mons[otmp->corpsenm]))
             otmp = otmp->nexthere;
         if (otmp != 0 || ((mtmp == &gy.youmonst) ? Stone_resistance
-                                                : resists_ston(mtmp)))
+                                                 : resists_ston(mtmp)))
             undetected = TRUE;
     }
 
@@ -4217,8 +4227,13 @@ hideunder(struct monst *mtmp)
         oldundetctd = u.uundetected != 0;
         u.uundetected = undetected ? 1 : 0;
     } else {
+        if (seeit)
+            seenmon = y_monnam(mtmp);
         oldundetctd = mtmp->mundetected != 0;
         mtmp->mundetected = undetected ? 1 : 0;
+        if (undetected && seenmon && seenobj)
+            You_see("%s %s under %s", seenmon,
+                    locomotion(mtmp->data, "hide"), seenobj);
     }
     if (undetected != oldundetctd)
         newsym(x, y);