expand implicit fallthrough detection to non-gcc compilers

gcc has recognized various "magic comments" for white-listing
occurrences of implicit fallthrough in switch statements for
a long time:

    The range and shape of "falls through" comments accepted are
    contingent upon the level of the warning. (The default level is =3.)

    -Wimplicit-fallthrough=0 disables the warning altogether.
    -Wimplicit-fallthrough=1 treats any kind of comment as a "falls through" comment.
    -Wimplicit-fallthrough=2 essentially accepts any comment that contains something
     that matches (case insensitively) "falls?[ \t-]*thr(ough|u)" regular expression.
    -Wimplicit-fallthrough=3 case sensitively matches a wide range of regular
     expressions, listed in the GCC manual. E.g., all of these are accepted:
        /* Falls through. */
        /* fall-thru */
        /* Else falls through. */
        /* FALLTHRU */
        /* ... falls through ... */
       etc.
    -Wimplicit-fallthrough=4 also, case sensitively matches a range of regular
     expressions but is much more strict than level =3.
    -Wimplicit-fallthrough=5 doesn't recognize any comments.

Plenty of other compilers did not recognize the gcc comment convention,
and up until now the compiler warning for detecting unintended
fallthrough had to be suppressed on other compilers. That's because the code
in NetHack has been relying on the gcc approach, and only the gcc approach.

The C23 standard introduces an attribute [[fallthrough]] for the
functionality, when implicit fallthrough warnings have been enabled.

Several popular compilers already support that, or a very similar attribute
style approach, today, even ahead of their C23 support:

       C compiler                       whitelist approach
       ---------------------------   -------------------------------------
       C23 conforming compilers         [[fallthrough]]

       clang versions supporting
       standards prior to
       C23                              __attribute__((__fallthrough__))

       Microsoft Visual Studio
       since VS 2022 17.4.
       The warning C5262 controls
       whether the implict
       fallthrough is detected and
       warned about with
       /std:clatest.                    [[fallthrough]]

This adds support to NetHack for the attribute approach by inserting a
macro FALLTHROUGH to the existing cases that require white-listing, so
other compilers can analyze things too.

The definition of the FALLTHROUGH macro is controlled in include/tradstdc.h.

The gcc comment approach has also been left in place at this time.

include/tradstdc.h

@@ -327,13 +327,20 @@ typedef genericptr genericptr_t; /* (void *) or (char *) */
 /*
  * Give first priority to standard
  */
-#ifndef ATTRNORETURN
 #if defined(__STDC_VERSION__) || defined(__cplusplus)
 #if (__STDC_VERSION__ > 202300L) || defined(__cplusplus)
+#ifndef ATTRNORETURN
 #define ATTRNORETURN [[noreturn]]
 #endif
-#endif
+#ifndef __has_c_attribute
-#endif
+#define __has_c_attribute(x) 0
+#endif /* __has_c_attribute */
+#if __has_c_attribute(fallthrough)
+/* Standard attribute is available, use it. */
+#define FALLTHROUGH [[fallthrough]]
+#endif  /* __has_c_attribute(fallthrough) */
+#endif  /* __STDC_VERSION__ gt 202300L || __cplusplus */
+#endif  /* __STDC_VERSION || __cplusplus */
 
 /*
  * Allow gcc2 to check parameters of printf-like calls with -Wformat;
@@ -366,12 +373,21 @@ typedef genericptr genericptr_t; /* (void *) or (char *) */
 #endif  /* !NONNULLS_DEFINED */
 /* #pragma message is available */
 #define NH_PRAGMA_MESSAGE 1
-#endif
+#endif  /* __GNUC__ greater than or equal to 5 */
-#endif
+#endif  /* __GNUC__ */
 
-#if defined(__clang__) && !defined(DO_DEFINE_NONNULLS)
+#if defined(__clang__)
+#ifndef FALLTHROUGH
+#if defined(__clang_major__)
+#if __clang_major__ >= 9
+#define FALLTHROUGH __attribute__((fallthrough))
+#endif  /* __clang_major__ greater than or equal to 9 */
+#endif  /* __clang_major__ is defined */
+#endif  /* FALLTHROUGH */
+#if !defined(DO_DEFINE_NONNULLS)
 #define DO_DEFINE_NONNULLS
 #endif
+#endif  /* __clang__ */
 
 #if defined(DO_DEFINE_NONNULLS) && !defined(NONNULLS_DEFINED)
 #define NONNULL __attribute__((returns_nonnull))
@@ -405,6 +421,7 @@ typedef genericptr genericptr_t; /* (void *) or (char *) */
 #define NH_PRAGMA_MESSAGE 1
 #endif
 
+/* Fallback implementations */
 #ifndef PRINTF_F
 #define PRINTF_F(f, v)
 #endif
@@ -414,6 +431,9 @@ typedef genericptr genericptr_t; /* (void *) or (char *) */
 #ifndef UNUSED
 #define UNUSED
 #endif
+#ifndef FALLTHROUGH
+#define FALLTHROUGH
+#endif
 #ifndef ATTRNORETURN
 #define ATTRNORETURN
 #endif

src/allmain.c

@@ -1040,6 +1040,7 @@ argcheck(int argc, char *argv[], enum earlyarg e_arg)
                 extended_opt++;
                 return windows_early_options(extended_opt);
             }
+        FALLTHROUGH;
         /*FALLTHRU*/
 #endif
         default:

src/apply.c

@@ -3772,6 +3772,7 @@ use_grapple(struct obj *obj)
             (void) thitmonst(mtmp, uwep);
             return ECMD_TIME;
         }
+        FALLTHROUGH;
     /*FALLTHRU*/
     case 3: /* Surface */
         if (IS_AIR(levl[cc.x][cc.y].typ) || is_pool(cc.x, cc.y))
@@ -3891,6 +3892,7 @@ do_break_wand(struct obj *obj)
             discard_broken_wand();
             return ECMD_TIME;
         }
+        FALLTHROUGH;
         /*FALLTHRU*/
     case WAN_WISHING:
     case WAN_NOTHING:
@@ -3919,6 +3921,7 @@ do_break_wand(struct obj *obj)
         Soundeffect(se_wall_of_force, 65);
         pline("A wall of force smashes down around you!");
         dmg = d(1 + obj->spe, 6); /* normally 2d12 */
+        FALLTHROUGH;
         /*FALLTHRU*/
     case WAN_CANCELLATION:
     case WAN_POLYMORPH:
@@ -4304,6 +4307,7 @@ doapply(void)
             pline("It rings! ... But no-one answers.");
             break;
         }
+        FALLTHROUGH;
         /*FALLTHRU*/
     default:
         /* Pole-weapons can strike at a distance */

src/ball.c

@@ -743,7 +743,8 @@ drag_ball(coordxy x, coordxy y, int *bc_control,
                     SKIP_TO_DRAG;
                 break;
             }
-        /* fall through */
+	    FALLTHROUGH;
+        /* FALLTHRU */
         case 1:
         case 0:
             /* do nothing if possible */

src/dbridge.c

@@ -255,6 +255,7 @@ create_drawbridge(coordxy x, coordxy y, int dir, boolean flag)
         break;
     default:
         impossible("bad direction in create_drawbridge");
+        FALLTHROUGH;
         /*FALLTHRU*/
     case DB_WEST:
         horiz = FALSE;

src/display.c

@@ -528,6 +528,7 @@ display_monster(
         default:
             impossible("display_monster:  bad m_ap_type value [ = %d ]",
                        (int) mon->m_ap_type);
+            FALLTHROUGH;
             /*FALLTHRU*/
         case M_AP_NOTHING:
             show_glyph(x, y, mon_to_glyph(mon, newsym_rn2));
@@ -3566,6 +3567,7 @@ wall_angle(struct rm *lev)
     case SDOOR:
         if (lev->horizontal)
             goto horiz;
+        FALLTHROUGH;
         /*FALLTHRU*/
     case VWALL:
         switch (lev->wall_info & WM_MASK) {

src/do.c

@@ -2217,6 +2217,7 @@ revive_corpse(struct obj *corpse)
                 fill_pit(mtmp->mx, mtmp->my);
                 break;
             }
+            FALLTHROUGH;
             /*FALLTHRU*/
         default:
             /* we should be able to handle the other cases... */

src/do_name.c

@@ -442,6 +442,7 @@ objtyp_is_callable(int i)
            determine which one was the real one */
         if (i == AMULET_OF_YENDOR || i == FAKE_AMULET_OF_YENDOR)
             break; /* return FALSE */
+        FALLTHROUGH;
         /*FALLTHRU*/
     case SCROLL_CLASS:
     case POTION_CLASS:

src/do_wear.c

@@ -452,6 +452,7 @@ Helmet_on(void)
                          : (uarmh->o_id % 2) ? A_CHAOTIC : A_LAWFUL,
                      A_CG_HELM_ON);
         /* makeknown(HELM_OF_OPPOSITE_ALIGNMENT); -- below, after Tobjnam() */
+        FALLTHROUGH;
     /*FALLTHRU*/
     case DUNCE_CAP:
         if (uarmh && !uarmh->cursed) {

src/dog.c

@@ -523,7 +523,9 @@ mon_arrive(struct monst *mtmp, int when)
         } else if (!(u.uevent.qexpelled
                      && (Is_qstart(&u.uz0) || Is_qstart(&u.uz)))) {
             impossible("mon_arrive: no corresponding portal?");
-        } /*FALLTHRU*/
+        }
+        FALLTHROUGH;
+        /*FALLTHRU*/
     default:
     case MIGR_RANDOM:
         xlocale = ylocale = 0;
@@ -1076,6 +1078,7 @@ dogfood(struct monst *mon, struct obj *obj)
             && obj->oclass != BALL_CLASS
             && obj->oclass != CHAIN_CLASS)
             return APPORT;
+        FALLTHROUGH;
         /*FALLTHRU*/
     case ROCK_CLASS:
         return UNDEF;

src/dogmove.c

@@ -76,6 +76,7 @@ droppables(struct monst *mon)
             if (pickaxe && pickaxe->otyp == PICK_AXE && pickaxe != wep
                 && (!pickaxe->oartifact || obj->oartifact))
                 return pickaxe; /* drop the one we earlier decided to keep */
+            FALLTHROUGH;
         /*FALLTHRU*/
         case PICK_AXE:
             if (!pickaxe || (obj->oartifact && !pickaxe->oartifact)) {
@@ -104,12 +105,14 @@ droppables(struct monst *mon)
             if (key && key->otyp == LOCK_PICK
                 && (!key->oartifact || obj->oartifact))
                 return key; /* drop the one we earlier decided to keep */
+            FALLTHROUGH;
         /*FALLTHRU*/
         case LOCK_PICK:
             /* keep lock-pick in preference to credit card */
             if (key && key->otyp == CREDIT_CARD
                 && (!key->oartifact || obj->oartifact))
                 return key;
+            FALLTHROUGH;
         /*FALLTHRU*/
         case CREDIT_CARD:
             if (!key || (obj->oartifact && !key->oartifact)) {

src/dokick.c

@@ -1340,6 +1340,7 @@ dokick(void)
                 pline("%s burps loudly.", Monnam(u.ustuck));
                 break;
             }
+            FALLTHROUGH;
             /*FALLTHRU*/
         default:
             Your("feeble kick has no effect.");
@@ -1484,6 +1485,7 @@ drop_to(coord *cc, schar loc, coordxy x, coordxy y)
             cc->y = cc->x = 0;
             break;
         }
+        FALLTHROUGH;
         /*FALLTHRU*/
     case MIGR_STAIRS_UP:
     case MIGR_LADDER_UP:
@@ -1800,6 +1802,7 @@ obj_delivery(boolean near_hero)
         switch (where) {
         case MIGR_LADDER_UP:
             isladder = TRUE;
+            FALLTHROUGH;
             /*FALLTHRU*/
         case MIGR_STAIRS_UP:
         case MIGR_SSTAIRS:

src/dothrow.c

@@ -67,6 +67,7 @@ multishot_class_bonus(
     case PM_NINJA:
         if (skill == -P_SHURIKEN || skill == -P_DART)
             multishot++;
+        FALLTHROUGH;
         /*FALLTHRU*/
     case PM_SAMURAI:
         /* role-specific launcher and its ammo */
@@ -175,6 +176,7 @@ throw_obj(struct obj *obj, int shotlimit)
         switch (P_SKILL(weapon_type(obj))) {
         case P_EXPERT:
             multishot++;
+            FALLTHROUGH;
         /*FALLTHRU*/
         case P_SKILLED:
             if (!weakmultishot)
@@ -1295,6 +1297,7 @@ toss_up(struct obj *obj, boolean hitsroof)
                     Your("%s fails to protect you.", helm_simple_name(uarmh));
                 goto petrify;
             }
+            FALLTHROUGH;
             /*FALLTHRU*/
         case CREAM_PIE:
         case BLINDING_VENOM:
@@ -2572,12 +2575,14 @@ breakmsg(struct obj *obj, boolean in_view)
     default: /* glass or crystal wand */
         if (obj->oclass != WAND_CLASS)
             impossible("breaking odd object (%d)?", obj->otyp);
+        FALLTHROUGH;
         /*FALLTHRU*/
     case LENSES:
     case MIRROR:
     case CRYSTAL_BALL:
     case EXPENSIVE_CAMERA:
         to_pieces = " into a thousand pieces";
+        FALLTHROUGH;
     /*FALLTHRU*/
     case POT_WATER: /* really, all potions */
         if (!in_view)

src/dungeon.c

@@ -3039,6 +3039,7 @@ count_feat_lastseentyp(
         }
         if (is_drawbridge_wall(x, y) < 0)
             break;
+        FALLTHROUGH;
         /*FALLTHRU*/
     case DBWALL:
     case DRAWBRIDGE_DOWN:

src/eat.c

@@ -848,6 +848,7 @@ cprefx(int pm)
             make_slimed(10L, (char *) 0);
             delayed_killer(SLIMED, KILLED_BY_AN, "");
         }
+        FALLTHROUGH;
     /* Fall through */
     default:
         if (acidic(&mons[pm]) && Stoned)
@@ -1164,19 +1165,23 @@ cpostfx(int pm)
             HSee_invisible |= FROMOUTSIDE;
         }
         newsym(u.ux, u.uy);
+        FALLTHROUGH;
         /*FALLTHRU*/
     case PM_YELLOW_LIGHT:
     case PM_GIANT_BAT:
         make_stunned((HStun & TIMEOUT) + 30L, FALSE);
+        FALLTHROUGH;
         /*FALLTHRU*/
     case PM_BAT:
         make_stunned((HStun & TIMEOUT) + 30L, FALSE);
         break;
     case PM_GIANT_MIMIC:
         tmp += 10;
+        FALLTHROUGH;
         /*FALLTHRU*/
     case PM_LARGE_MIMIC:
         tmp += 20;
+        FALLTHROUGH;
         /*FALLTHRU*/
     case PM_SMALL_MIMIC:
         tmp += 20;
@@ -1278,6 +1283,7 @@ cpostfx(int pm)
         } else {
             pline("For some reason, that tasted bland.");
         }
+        FALLTHROUGH;
     /*FALLTHRU*/
     default:
         check_intrinsics = TRUE;
@@ -2143,6 +2149,7 @@ fprefx(struct obj *otmp)
             break;
         }
         iter_mons(garlic_breath);
+        FALLTHROUGH;
         /*FALLTHRU*/
     default:
         if (otmp->otyp == SLIME_MOLD && !otmp->cursed

src/engrave.c

@@ -606,6 +606,7 @@ doengrave_sfx_item_WAN(struct _doengrave_ctx *de)
                    "A few ice cubes drop from the wand.");
         if (!de->oep || (de->oep->engr_type != BURN))
             break;
+        FALLTHROUGH;
         /*FALLTHRU*/
     case WAN_CANCELLATION:
     case WAN_MAKE_INVISIBLE:
@@ -706,6 +707,7 @@ doengrave_sfx_item(struct _doengrave_ctx *de)
             de->type = DUST;
             break;
         }
+        FALLTHROUGH;
         /*FALLTHRU*/
     /* Objects too large to engrave with */
     case BALL_CLASS:

src/fountain.c

@@ -359,6 +359,7 @@ drinkfountain(void)
                 dofindgem();
                 break;
             }
+            FALLTHROUGH;
             /*FALLTHRU*/
         case 28: /* Water Nymph */
             dowaternymph();
@@ -486,6 +487,7 @@ dipfountain(struct obj *obj)
             dofindgem();
             break;
         }
+        FALLTHROUGH;
         /*FALLTHRU*/
     case 25: /* Water gushes forth */
         dogushforth(FALSE);
@@ -699,6 +701,7 @@ drinksink(void)
             pline("From the murky drain, a hand reaches up... --oops--");
             break;
         }
+        FALLTHROUGH;
         /*FALLTHRU*/
     default:
         You("take a sip of %s %s.",
@@ -770,6 +773,7 @@ dipsink(struct obj *obj)
             try_call = TRUE;
             break;
         }
+        FALLTHROUGH;
         /* FALLTHRU */
     case POT_GAIN_LEVEL:
     case POT_GAIN_ENERGY:

src/getpos.c

@@ -465,6 +465,7 @@ gather_locs_interesting(coordxy x, coordxy y, int gloc)
     case GLOC_VALID:
         if (getpos_getvalid)
             return (*getpos_getvalid)(x, y);
+        FALLTHROUGH;
         /*FALLTHRU*/
     case GLOC_INTERESTING:
         return (gather_locs_interesting(x, y, GLOC_DOOR)

src/hack.c

@@ -571,6 +571,7 @@ moverock_core(coordxy sx, coordxy sy)
                         dopush(sx, sy, rx, ry, otmp, costly);
                         continue;
                     }
+                    FALLTHROUGH;
                     /*FALLTHRU*/
                 case TELEP_TRAP:
                     rock_disappear_msg(otmp);
@@ -2553,6 +2554,7 @@ escape_from_sticky_mon(coordxy x, coordxy y)
                     u.ustuck->mfrozen = 1;
                     u.ustuck->msleeping = 0;
                 }
+                FALLTHROUGH;
                 /*FALLTHRU*/
             default:
                 if (u.ustuck->mtame && !Conflict && !u.ustuck->mconf)
@@ -3573,6 +3575,7 @@ check_special_room(boolean newlev)
         }
         case TEMPLE:
             intemple(roomno + ROOMOFFSET);
+            FALLTHROUGH;
         /*FALLTHRU*/
         default:
             msg_given = (rt == TEMPLE || rt >= SHOPBASE);
@@ -4317,6 +4320,7 @@ spot_checks(coordxy x, coordxy y, schar old_typ)
     switch (old_typ) {
     case DRAWBRIDGE_UP:
         db_ice_now = ((levl[x][y].drawbridgemask & DB_UNDER) == DB_ICE);
+        FALLTHROUGH;
         /*FALLTHRU*/
     case ICE:
         if ((new_typ != old_typ)

src/insight.c

@@ -1968,6 +1968,8 @@ attributes_enlightenment(
             switch (u.umortality) {
             case 0:
                 impossible("dead without dying?");
+                FALLTHROUGH;
+                /* FALLTHRU */
             case 1:
                 break; /* just "are dead" */
             default:
@@ -2624,6 +2626,7 @@ vanqsort_cmp(
             res = uniq2 - uniq1;
             break;
         } /* else both unique or neither unique */
+        FALLTHROUGH;
         /*FALLTHRU*/
     case VANQ_ALPHA_MIX:
         name1 = mons[indx1].pmnames[NEUTRAL];

src/invent.c

@@ -2463,6 +2463,7 @@ askchain(
         switch (sym) {
         case 'a':
             allflag = 1;
+            FALLTHROUGH;
             /*FALLTHRU*/
         case 'y':
             tmp = (*fn)(otmp);
@@ -2481,10 +2482,13 @@ askchain(
             cnt += tmp;
             if (--mx == 0)
                 goto ret;
+            FALLTHROUGH;
             /*FALLTHRU*/
         case 'n':
             if (nodot)
                 dud++;
+            FALLTHROUGH;
+            /*FALLTHRU*/
         default:
             break;
         case 'q':
@@ -2975,6 +2979,7 @@ itemactions_pushkeys(struct obj *otmp, int act)
         switch (act) {
         default:
             impossible("Unknown item action");
+            break;
         case IA_NONE:
             break;
         case IA_UNWIELD:

src/makemon.c

@@ -521,6 +521,7 @@ m_initweap(struct monst *mtmp)
          */
         if (!is_demon(ptr))
             break;
+        FALLTHROUGH;
         /*FALLTHRU*/
     default:
         /*
@@ -704,12 +705,15 @@ m_initinv(struct monst *mtmp)
             /* MAJOR fall through ... */
             case 0:
                 (void) mongets(mtmp, WAN_MAGIC_MISSILE);
+                FALLTHROUGH;
                 /*FALLTHRU*/
             case 1:
                 (void) mongets(mtmp, POT_EXTRA_HEALING);
+                FALLTHROUGH;
                 /*FALLTHRU*/
             case 2:
                 (void) mongets(mtmp, POT_HEALING);
+                FALLTHROUGH;
                 /*FALLTHRU*/
             case 3:
                 (void) mongets(mtmp, WAN_STRIKING);

src/mcastu.c

@@ -84,6 +84,7 @@ choose_magic_spell(int spellval)
     case 23:
         if (Antimagic || Hallucination)
             return MGC_PSI_BOLT;
+        FALLTHROUGH;
         /*FALLTHRU*/
     case 22:
     case 21:
@@ -138,6 +139,7 @@ choose_clerical_spell(int spellnum)
     case 14:
         if (rn2(3))
             return CLC_OPEN_WOUNDS;
+        FALLTHROUGH;
         /*FALLTHRU*/
     case 13:
         return CLC_GEYSER;

src/mhitm.c

@@ -405,6 +405,7 @@ mattackm(
                     mswingsm(magr, mdef, mwep);
                 tmp += hitval(mwep, mdef);
             }
+            FALLTHROUGH;
             /*FALLTHRU*/
         case AT_CLAW:
         case AT_KICK:
@@ -683,6 +684,7 @@ hitmm(
                     Snprintf(buf, sizeof buf, "%s squeezes", magr_name);
                     break;
                 }
+                FALLTHROUGH;
                 /*FALLTHRU*/
             default:
                 if (!weaponhit || !mwep || !mwep->oartifact)

src/mkmaze.c

@@ -588,6 +588,7 @@ fixup_special(void)
                 sp = find_level(r->rname.str);
                 lev = sp->dlevel;
             }
+            FALLTHROUGH;
             /*FALLTHRU*/
 
         case LR_UPSTAIR:
@@ -2073,6 +2074,7 @@ mv_bubble(struct bubble *b, coordxy dx, coordxy dy, boolean ini)
         break;
     case 3:
         b->dy = -b->dy;
+        FALLTHROUGH;
         /*FALLTHRU*/
     case 2:
         b->dx = -b->dx;

src/mkobj.c

@@ -325,6 +325,7 @@ mkbox_cnts(struct obj *box)
             n = 0;
             break;
         }
+        FALLTHROUGH;
         /*FALLTHRU*/
     case BAG_OF_HOLDING:
         n = 1;
@@ -999,6 +1000,7 @@ mksobj_init(struct obj *otmp, boolean artif)
         case LARGE_BOX:
             otmp->olocked = !!(rn2(5));
             otmp->otrapped = !(rn2(10));
+            FALLTHROUGH;
             /*FALLTHRU*/
         case ICE_BOX:
         case SACK:
@@ -1184,6 +1186,7 @@ mksobj(int otyp, boolean init, boolean artif)
             if (svm.mvitals[otmp->corpsenm].mvflags & (G_NOCORPSE | G_GONE))
                 otmp->corpsenm = gu.urole.mnum;
         }
+        FALLTHROUGH;
         /*FALLTHRU*/
     case STATUE:
     case FIGURINE:
@@ -1197,6 +1200,7 @@ mksobj(int otyp, boolean init, boolean artif)
                            : is_male(ptr) ? CORPSTAT_MALE
                              : rn2(2) ? CORPSTAT_FEMALE : CORPSTAT_MALE);
         }
+        FALLTHROUGH;
         /*FALLTHRU*/
     case EGG:
     /* case TIN: */
@@ -1210,6 +1214,7 @@ mksobj(int otyp, boolean init, boolean artif)
         break;
     case POT_OIL:
         otmp->age = MAX_OIL_IN_FLASK; /* amount of oil */
+        FALLTHROUGH;
         /*FALLTHRU*/
     case POT_WATER: /* POTION_CLASS */
         otmp->fromsink = 0; /* overloads corpsenm, which was set to NON_PM */
@@ -2982,6 +2987,7 @@ objlist_sanity(struct obj *objlist, int wheretype, const char *mesg)
                 /* note: ball and chain can also be OBJ_FREE, but not across
                    turns so this sanity check shouldn't encounter that */
                 bc_ok = TRUE;
+            FALLTHROUGH;
             /*FALLTHRU*/
             default:
                 if ((obj != uchain && obj != uball) || !bc_ok) {

src/mon.c

@@ -861,7 +861,6 @@ make_corpse(struct monst *mtmp, unsigned int corpseflags)
     case PM_PAGE: case PM_ABBOT: case PM_ACOLYTE: case PM_HUNTER:
     case PM_THUG: case PM_NINJA: case PM_ROSHI: case PM_GUIDE:
     case PM_WARRIOR: case PM_APPRENTICE:
-    /*FALLTHRU*/
 #else
     default:
 #endif
@@ -3076,7 +3075,8 @@ mondead(struct monst *mtmp)
                 (void) makemon(mtmp->data, stway->sx, stway->sy, NO_MM_FLAGS);
                 break;
             }
-            /* fall-through */
+	    FALLTHROUGH;
+            /* FALLTHRU */
         case 2: /* randomly */
             (void) makemon(mtmp->data, 0, 0, NO_MM_FLAGS);
             break;
@@ -4791,12 +4791,14 @@ pickvampshape(struct monst *mon)
         if (mon_has_special(mon))
             break; /* leave mndx as is */
         wolfchance = 3;
+        FALLTHROUGH;
     /*FALLTHRU*/
     case PM_VAMPIRE_LEADER: /* vampire lord or Vlad can become wolf */
         if (!rn2(wolfchance) && !uppercase_only) {
             mndx = PM_WOLF;
             break;
         }
+        FALLTHROUGH;
     /*FALLTHRU*/
     case PM_VAMPIRE: /* any vampire can become fog or bat */
         mndx = (!rn2(4) && !uppercase_only) ? PM_FOG_CLOUD : PM_VAMPIRE_BAT;
@@ -4900,6 +4902,7 @@ validvamp(struct monst *mon, int *mndx_p, int monclass)
             *mndx_p = PM_WOLF;
             break;
         }
+        FALLTHROUGH;
         /*FALLTHRU*/
     default:
         *mndx_p = NON_PM;

src/monmove.c

@@ -887,6 +887,7 @@ dochug(struct monst *mtmp)
         case MMOVE_NOMOVES:
             if (scared)
                 panicattk = TRUE;
+            FALLTHROUGH;
             /*FALLTHRU*/
         case MMOVE_NOTHING: /* no movement, but it can still attack you */
         case MMOVE_DONE: /* absolutely no movement */
@@ -1758,6 +1759,7 @@ m_move(struct monst *mtmp, int after)
             break;
         default:
             impossible("unknown shk/gd/pri_move return value (%d)", xm);
+            FALLTHROUGH;
             /*FALLTHRU*/
         case 0:
         case 1:

src/mthrowu.c

@@ -655,6 +655,7 @@ m_throw(
                     hitu = 0;
                     break;
                 }
+                FALLTHROUGH;
                 /*FALLTHRU*/
             case CREAM_PIE:
             case BLINDING_VENOM:
@@ -851,6 +852,7 @@ spitmm(struct monst *mtmp, struct attack *mattk, struct monst *mtarg)
             break;
         default:
             impossible("bad attack type in spitmm");
+            FALLTHROUGH;
             /*FALLTHRU*/
         case AD_ACID:
             otmp = mksobj(ACID_VENOM, TRUE, FALSE);

src/muse.c

@@ -1201,6 +1201,7 @@ rnd_defensive_item(struct monst *mtmp)
             goto try_again;
         if (!rn2(3))
             return WAN_TELEPORTATION;
+        FALLTHROUGH;
         /*FALLTHRU*/
     case 0:
     case 1:
@@ -1209,6 +1210,7 @@ rnd_defensive_item(struct monst *mtmp)
     case 10:
         if (!rn2(3))
             return WAN_CREATE_MONSTER;
+        FALLTHROUGH;
         /*FALLTHRU*/
     case 2:
         return SCR_CREATE_MONSTER;
@@ -1968,7 +1970,9 @@ rnd_offensive_item(struct monst *mtmp)
         if (hard_helmet(mtmp_helmet) || amorphous(pm)
             || passes_walls(pm) || noncorporeal(pm) || unsolid(pm))
             return SCR_EARTH;
-    } /* fall through */
+    }
+    FALLTHROUGH;
+    /* FALLTHRU */
     case 1:
         return WAN_STRIKING;
     case 2:

src/music.c

@@ -443,6 +443,7 @@ do_earthquake(int force)
                 unblock_point(x, y);
                 if (cansee(x, y))
                     pline("A secret corridor is revealed.");
+                FALLTHROUGH;
                 /*FALLTHRU*/
             case CORR:
             case ROOM:
@@ -452,6 +453,7 @@ do_earthquake(int force)
                 cvt_sdoor_to_door(&levl[x][y]); /* .typ = DOOR */
                 if (cansee(x, y))
                     pline("A secret door is revealed.");
+                FALLTHROUGH;
                 /*FALLTHRU*/
             case DOOR: /* make the door collapse */
                 /* if already doorless, treat like room or corridor */

src/nhlua.c

@@ -1996,6 +1996,8 @@ nhl_pcall_handle(lua_State *L, int nargs, int nresults, const char *name,
         case NHLpa_panic:
             panic("Lua error %d:%s %s", nud->sid,
                   nud->name ? nud->name : "(unknown)", lua_tostring(L, -1));
+            /*NOTREACHED*/
+            break;
         case NHLpa_impossible:
             impossible("Lua error: %d:%s %s", nud->sid,
                        nud->name ? nud->name : "(unknown)",

src/objnam.c

@@ -677,6 +677,7 @@ xname_flags(
     case WEAPON_CLASS:
         if (is_poisonable(obj) && obj->opoisoned)
             Strcpy(buf, "poisoned ");
+        FALLTHROUGH;
         /*FALLTHRU*/
     case VENOM_CLASS:
     case TOOL_CLASS:
@@ -1391,6 +1392,7 @@ doname_base(
                     ConcatF1(bp, 1, ", %s lit)", arti_light_description(obj));
             }
         }
+        FALLTHROUGH;
         /*FALLTHRU*/
     case WEAPON_CLASS:
         if (ispoisoned)
@@ -5077,7 +5079,8 @@ readobjnam(char *bp, struct obj *no_wish)
         break;
     case SLIME_MOLD:
         d.otmp->spe = d.ftype;
-    /* Fall through */
+        FALLTHROUGH;
+    /* FALLTHRU */
     case SKELETON_KEY:
     case CHEST:
     case LARGE_BOX:
@@ -5109,7 +5112,8 @@ readobjnam(char *bp, struct obj *no_wish)
     /* scroll of mail:  0: delivered in-game via external event (or randomly
        for fake mail); 1: from bones or wishing; 2: written with marker */
     case SCR_MAIL:
-        /*FALLTHRU*/
+        d.otmp->spe = 1;
+	break;
 #endif
     /* splash of venom:  0: normal, and transitory; 1: wishing */
     case ACID_VENOM:
@@ -5121,6 +5125,7 @@ readobjnam(char *bp, struct obj *no_wish)
             d.otmp->spe = (rn2(10) ? -1 : 0);
             break;
         }
+        FALLTHROUGH;
         /*FALLTHRU*/
     default:
         d.otmp->spe = d.spe;

src/options.c

@@ -3624,6 +3624,7 @@ optfn_scores(
                                      allopt[optidx].name);
                     return optn_silenterr;
                 }
+                FALLTHROUGH;
                 /*FALLTHRU*/
             default:
                 config_error_add("Unknown %s parameter '%s'",

src/pager.c

@@ -765,6 +765,7 @@ lookat(coordxy x, coordxy y, char *buf, char *monbuf)
                 Strcpy(buf, "stone");
                 break;
             }
+            FALLTHROUGH;
             /*FALLTHRU*/
         default:
             Strcpy(buf, defsyms[symidx].explanation);

src/pickup.c

@@ -870,6 +870,7 @@ pickup(int what) /* should be a long */
                     lcount = (long) yn_number;
                     if (lcount > obj->quan)
                         lcount = obj->quan;
+                    FALLTHROUGH;
                     /*FALLTHRU*/
                 default: /* 'y' */
                     break;
@@ -1478,6 +1479,7 @@ query_category(
                         /* assert( n == 1 ); */
                         break; /* from switch */
                     }
+                    FALLTHROUGH;
                     /*FALLTHRU*/
                 case 'q':
                 default:

src/polyself.c

@@ -1450,6 +1450,7 @@ dospit(void)
             break;
         default:
             impossible("bad attack type in dospit");
+            FALLTHROUGH;
             /*FALLTHRU*/
         case AD_ACID:
             otmp = mksobj(ACID_VENOM, TRUE, FALSE);

src/potion.c

@@ -1721,16 +1721,19 @@ potionhit(struct monst *mon, struct obj *obj, int how)
         switch (obj->otyp) {
         case POT_FULL_HEALING:
             cureblind = TRUE;
+            FALLTHROUGH;
             /*FALLTHRU*/
         case POT_EXTRA_HEALING:
             if (!obj->cursed)
                 cureblind = TRUE;
+            FALLTHROUGH;
             /*FALLTHRU*/
         case POT_HEALING:
             if (obj->blessed)
                 cureblind = TRUE;
             if (mon->data == &mons[PM_PESTILENCE])
                 goto do_illness;
+            FALLTHROUGH;
             /*FALLTHRU*/
         case POT_RESTORE_ABILITY:
         case POT_GAIN_ABILITY:
@@ -1960,6 +1963,7 @@ potionbreathe(struct obj *obj)
         if (u.uhp < u.uhpmax)
             u.uhp++, disp.botl = TRUE;
         cureblind = TRUE;
+        FALLTHROUGH;
         /*FALLTHRU*/
     case POT_EXTRA_HEALING:
         if (Upolyd && u.mh < u.mhmax)
@@ -1968,6 +1972,7 @@ potionbreathe(struct obj *obj)
             u.uhp++, disp.botl = TRUE;
         if (!obj->cursed)
             cureblind = TRUE;
+        FALLTHROUGH;
         /*FALLTHRU*/
     case POT_HEALING:
         if (Upolyd && u.mh < u.mhmax)
@@ -2116,6 +2121,7 @@ mixtype(struct obj *o1, struct obj *o2)
     case POT_HEALING:
         if (o2typ == POT_SPEED)
             return POT_EXTRA_HEALING;
+        FALLTHROUGH;
         /*FALLTHRU*/
     case POT_EXTRA_HEALING:
     case POT_FULL_HEALING:
@@ -2123,6 +2129,7 @@ mixtype(struct obj *o1, struct obj *o2)
             return (o1typ == POT_HEALING) ? POT_EXTRA_HEALING
                    : (o1typ == POT_EXTRA_HEALING) ? POT_FULL_HEALING
                      : POT_GAIN_ABILITY;
+        FALLTHROUGH;
         /*FALLTHRU*/
     case UNICORN_HORN:
         switch (o2typ) {

src/pray.c

@@ -403,7 +403,8 @@ fix_worst_trouble(int trouble)
         break;
     case TROUBLE_STARVING:
         /* temporarily lost strength recovery now handled by init_uhunger() */
-        /*FALLTHRU*/
+        FALLTHROUGH;
+	/* FALLTHRU*/
     case TROUBLE_HUNGRY:
         Your("%s feels content.", body_part(STOMACH));
         init_uhunger();
@@ -745,7 +746,9 @@ angrygods(aligntyp resp_god)
             gods_angry(resp_god);
             punish((struct obj *) 0);
             break;
-        } /* else fall thru */
+        }
+	FALLTHROUGH;
+	/* FALLTHRU */
     case 4:
     case 5:
         gods_angry(resp_god);
@@ -1127,6 +1130,7 @@ pleased(aligntyp g_align)
         switch (min(action, 5)) {
         case 5:
             pat_on_head = 1;
+            FALLTHROUGH;
             /*FALLTHRU*/
         case 4:
             do
@@ -1137,8 +1141,9 @@ pleased(aligntyp g_align)
         case 3:
             /* up to 10 troubles */
             fix_worst_trouble(trouble);
+            FALLTHROUGH;
             /*FALLTHRU*/
-        case 2:
+	    case 2:
             /* up to 9 troubles */
             while ((trouble = in_trouble()) > 0 && (++tryct < 10))
                 fix_worst_trouble(trouble);
@@ -1234,6 +1239,7 @@ pleased(aligntyp g_align)
                     break;
                 }
             }
+            FALLTHROUGH;
             /*FALLTHRU*/
         case 2:
             if (!Blind)
@@ -1335,6 +1341,7 @@ pleased(aligntyp g_align)
                 gcrownu();
                 break;
             }
+            FALLTHROUGH;
             /*FALLTHRU*/
         case 6:
             give_spell();
@@ -2335,18 +2342,23 @@ maybe_turn_mon_iter(struct monst *mtmp)
                    than zombies. */
             case S_LICH:
                 xlev += 2;
+                FALLTHROUGH;
                 /*FALLTHRU*/
             case S_GHOST:
                 xlev += 2;
+                FALLTHROUGH;
                 /*FALLTHRU*/
             case S_VAMPIRE:
                 xlev += 2;
+                FALLTHROUGH;
                 /*FALLTHRU*/
             case S_WRAITH:
                 xlev += 2;
+                FALLTHROUGH;
                 /*FALLTHRU*/
             case S_MUMMY:
                 xlev += 2;
+                FALLTHROUGH;
                 /*FALLTHRU*/
             case S_ZOMBIE:
                 if (u.ulevel >= xlev && !resist(mtmp, '\0', 0, NOTELL)) {
@@ -2358,6 +2370,7 @@ maybe_turn_mon_iter(struct monst *mtmp)
                     }
                     break;
                 } /* else flee */
+                FALLTHROUGH;
                 /*FALLTHRU*/
             default:
                 monflee(mtmp, 0, FALSE, TRUE);
@@ -2656,6 +2669,7 @@ blocked_boulder(int dx, int dy)
         /* this is only approximate since multiple boulders might sink */
         if (is_pool_or_lava(nx, ny)) /* does its own isok() check */
             break; /* still need Sokoban check below */
+        FALLTHROUGH;
         /*FALLTHRU*/
     default:
         /* more than one boulder--blocked after they push the top one;

src/questpgr.c

@@ -374,6 +374,7 @@ convert_line(char *in_line, char *out_line)
                 /* pluralize */
                 case 'P':
                     gc.cvt_buf[0] = highc(gc.cvt_buf[0]);
+                    FALLTHROUGH;
                     /*FALLTHRU*/
                 case 'p':
                     Strcpy(gc.cvt_buf, makeplural(gc.cvt_buf));
@@ -382,6 +383,7 @@ convert_line(char *in_line, char *out_line)
                 /* append possessive suffix */
                 case 'S':
                     gc.cvt_buf[0] = highc(gc.cvt_buf[0]);
+                    FALLTHROUGH;
                     /*FALLTHRU*/
                 case 's':
                     Strcpy(gc.cvt_buf, s_suffix(gc.cvt_buf));
@@ -403,8 +405,9 @@ convert_line(char *in_line, char *out_line)
                 Strcat(cc, gc.cvt_buf);
                 cc += strlen(gc.cvt_buf);
                 break;
-            } /* else fall through */
+            }
-
+	    FALLTHROUGH;
+	    /* FALLTHRU */
         default:
             *cc++ = *c;
             break;

src/role.c

@@ -1357,6 +1357,7 @@ clearrolefilter(int which)
     switch (which) {
     case RS_filter:
         gr.rfilter.mask = 0; /* clear race, gender, and alignment filters */
+        FALLTHROUGH;
         /*FALLTHRU*/
     case RS_ROLE:
         for (i = 0; i < SIZE(roles) - 1; ++i)

src/rumors.c

@@ -563,6 +563,7 @@ outrumor(
         return;
     case BY_COOKIE:
         pline(fortune_msg);
+        FALLTHROUGH;
     /* FALLTHRU */
     case BY_PAPER:
         pline("It reads:");

src/selvar.c

@@ -582,6 +582,7 @@ selection_do_gradient(
     switch (gtyp) {
     default:
         impossible("Unrecognized gradient type! Defaulting to radial...");
+        FALLTHROUGH;
         /* FALLTHRU */
     case SEL_GRADIENT_RADIAL: {
         for (dx = 0; dx < COLNO; dx++)

src/shk.c

@@ -4068,6 +4068,7 @@ sellobj(
         switch (gs.sell_response ? gs.sell_response : nyaq(qbuf)) {
         case 'q':
             gs.sell_response = 'n';
+            FALLTHROUGH;
             /*FALLTHRU*/
         case 'n':
             if (container)
@@ -4078,6 +4079,7 @@ sellobj(
             break;
         case 'a':
             gs.sell_response = 'y';
+            FALLTHROUGH;
             /*FALLTHRU*/
         case 'y':
             if (container)

src/sit.c

@@ -158,6 +158,7 @@ throne_sit_effect(void)
                     default:
                     case 2: /* more than 1 eye */
                         eye = makeplural(eye);
+                        FALLTHROUGH;
                         /*FALLTHRU*/
                     case 1: /* one eye (Cyclops, floating eye) */
                         Your("%s %s...", eye, vtense(eye, "tingle"));
@@ -517,6 +518,7 @@ attrcurse(void)
             ret = FIRE_RES;
             break;
         }
+        FALLTHROUGH;
         /*FALLTHRU*/
     case 2:
         if (HTeleportation & INTRINSIC) {
@@ -525,6 +527,7 @@ attrcurse(void)
             ret = TELEPORT;
             break;
         }
+        FALLTHROUGH;
         /*FALLTHRU*/
     case 3:
         if (HPoison_resistance & INTRINSIC) {
@@ -533,6 +536,7 @@ attrcurse(void)
             ret = POISON_RES;
             break;
         }
+        FALLTHROUGH;
         /*FALLTHRU*/
     case 4:
         if (HTelepat & INTRINSIC) {
@@ -543,6 +547,7 @@ attrcurse(void)
             ret = TELEPAT;
             break;
         }
+        FALLTHROUGH;
         /*FALLTHRU*/
     case 5:
         if (HCold_resistance & INTRINSIC) {
@@ -551,6 +556,7 @@ attrcurse(void)
             ret = COLD_RES;
             break;
         }
+        FALLTHROUGH;
         /*FALLTHRU*/
     case 6:
         if (HInvis & INTRINSIC) {
@@ -559,6 +565,7 @@ attrcurse(void)
             ret = INVIS;
             break;
         }
+        FALLTHROUGH;
         /*FALLTHRU*/
     case 7:
         if (HSee_invisible & INTRINSIC) {
@@ -574,6 +581,7 @@ attrcurse(void)
             ret = SEE_INVIS;
             break;
         }
+        FALLTHROUGH;
         /*FALLTHRU*/
     case 8:
         if (HFast & INTRINSIC) {
@@ -582,6 +590,7 @@ attrcurse(void)
             ret = FAST;
             break;
         }
+        FALLTHROUGH;
         /*FALLTHRU*/
     case 9:
         if (HStealth & INTRINSIC) {
@@ -590,6 +599,7 @@ attrcurse(void)
             ret = STEALTH;
             break;
         }
+        FALLTHROUGH;
         /*FALLTHRU*/
     case 10:
         /* intrinsic protection is just disabled, not set back to 0 */
@@ -599,6 +609,7 @@ attrcurse(void)
             ret = PROTECTION;
             break;
         }
+        FALLTHROUGH;
         /*FALLTHRU*/
     case 11:
         if (HAggravate_monster & INTRINSIC) {
@@ -607,6 +618,7 @@ attrcurse(void)
             ret = AGGRAVATE_MONSTER;
             break;
         }
+        FALLTHROUGH;
         /*FALLTHRU*/
     default:
         break;

src/sounds.c

@@ -264,6 +264,7 @@ dosounds(void)
                     break;
                 }
             }
+            FALLTHROUGH;
                 /*FALLTHRU*/
             case 0:
                 Soundeffect(se_guards_footsteps, 30);
@@ -631,7 +632,6 @@ cry_sound(struct monst *mtmp)
         ret = "hiss";
         break;
     case MS_ROAR: /* baby dragons; have them growl instead of roar */
-        /*FALLTHRU*/
     case MS_GROWL: /* (none) */
         ret = "growl";
         break;
@@ -870,6 +870,7 @@ domonnoise(struct monst *mtmp)
             }
             break;
         }
+        FALLTHROUGH;
         /*FALLTHRU*/
     case MS_GROWL:
         Soundeffect((mtmp->mpeaceful ? se_snarl : se_growl), 80);
@@ -1019,6 +1020,7 @@ domonnoise(struct monst *mtmp)
             }
             break;
         }
+        FALLTHROUGH;
         /*FALLTHRU*/
     case MS_HUMANOID:
         if (!mtmp->mpeaceful) {
@@ -1141,7 +1143,8 @@ domonnoise(struct monst *mtmp)
             (void) demon_talk(mtmp);
             break;
         }
-    /* fall through */
+	FALLTHROUGH;
+        /* FALLTHRU */
     case MS_CUSS:
         if (!mtmp->mpeaceful)
             cuss(mtmp);

src/spell.c

@@ -1441,11 +1441,13 @@ spelleffects(int spell_otyp, boolean atme, boolean force)
             }
             break;
         } /* else */
+        FALLTHROUGH;
         /*FALLTHRU*/
 
     /* these spells are all duplicates of wand effects */
     case SPE_FORCE_BOLT:
         physical_damage = TRUE;
+        FALLTHROUGH;
     /*FALLTHRU*/
     case SPE_SLEEP:
     case SPE_MAGIC_MISSILE:
@@ -1510,6 +1512,7 @@ spelleffects(int spell_otyp, boolean atme, boolean force)
         /* high skill yields effect equivalent to blessed scroll */
         if (role_skill >= P_SKILLED)
             pseudo->blessed = 1;
+        FALLTHROUGH;
     /*FALLTHRU*/
     case SPE_CHARM_MONSTER:
     case SPE_MAGIC_MAPPING:
@@ -1526,6 +1529,7 @@ spelleffects(int spell_otyp, boolean atme, boolean force)
         /* high skill yields effect equivalent to blessed potion */
         if (role_skill >= P_SKILLED)
             pseudo->blessed = 1;
+        FALLTHROUGH;
     /*FALLTHRU*/
     case SPE_INVISIBILITY:
         (void) peffects(pseudo);

src/steed.c

@@ -589,6 +589,7 @@ dismount_steed(
     switch (reason) {
     case DISMOUNT_THROWN:
         verb = "are thrown";
+        FALLTHROUGH;
         /*FALLTHRU*/
     case DISMOUNT_KNOCKED:
     case DISMOUNT_FELL:

src/timeout.c

@@ -214,6 +214,7 @@ vomiting_dialogue(void)
         make_stunned((HStun & TIMEOUT) + (long) d(2, 4), FALSE);
         if (!Popeye(VOMITING))
             stop_occupation();
+        FALLTHROUGH;
     /*FALLTHRU*/
     case 9:
         make_confused((HConfusion & TIMEOUT) + (long) d(2, 4), FALSE);
@@ -1441,6 +1442,7 @@ burn_object(anything *arg, long timeout)
             switch (obj->where) {
             case OBJ_INVENT:
                 need_invupdate = TRUE;
+                FALLTHROUGH;
                 /*FALLTHRU*/
             case OBJ_MINVENT:
                 pline("%spotion of oil has burnt away.", whose);
@@ -1504,6 +1506,7 @@ burn_object(anything *arg, long timeout)
                 switch (obj->where) {
                 case OBJ_INVENT:
                     need_invupdate = TRUE;
+                    FALLTHROUGH;
                     /*FALLTHRU*/
                 case OBJ_MINVENT:
                     if (obj->otyp == BRASS_LANTERN)
@@ -1583,6 +1586,7 @@ burn_object(anything *arg, long timeout)
                     switch (obj->where) {
                     case OBJ_INVENT:
                         need_invupdate = TRUE;
+                        FALLTHROUGH;
                         /*FALLTHRU*/
                     case OBJ_MINVENT:
                         pline("%scandelabrum's flame%s.", whose,
@@ -1598,6 +1602,7 @@ burn_object(anything *arg, long timeout)
                     case OBJ_INVENT:
                         /* no need_invupdate for update_inventory() necessary;
                            useupall() -> freeinv() handles it */
+                           FALLTHROUGH;
                         /*FALLTHRU*/
                     case OBJ_MINVENT:
                         pline("%s %s consumed!", Yname2(obj),

src/topten.c

@@ -110,11 +110,13 @@ formatkiller(
     switch (svk.killer.format) {
     default:
         impossible("bad killer format? (%d)", svk.killer.format);
+        FALLTHROUGH;
         /*FALLTHRU*/
     case NO_KILLER_PREFIX:
         break;
     case KILLED_BY_AN:
         kname = an(kname);
+        FALLTHROUGH;
         /*FALLTHRU*/
     case KILLED_BY:
         (void) strncat(buf, killed_by_prefix[how], siz - 1);

src/trap.c

@@ -513,6 +513,7 @@ maketrap(coordxy x, coordxy y, int typ)
     case PIT:
     case SPIKED_PIT:
         ttmp->conjoined = 0;
+        FALLTHROUGH;
         /*FALLTHRU*/
     case HOLE:
     case TRAPDOOR:
@@ -1126,10 +1127,13 @@ m_harmless_trap(struct monst *mtmp, struct trap *ttmp)
             return TRUE;
         break;
     case PIT:
+        FALLTHROUGH;
         /*FALLTHRU*/
     case SPIKED_PIT:
+        FALLTHROUGH;
         /*FALLTHRU*/
     case HOLE:
+        FALLTHROUGH;
         /*FALLTHRU*/
     case TRAPDOOR:
         if (is_clinger(mdat) && !Sokoban)
@@ -2190,6 +2194,7 @@ trapeffect_web(
                 mtmp->mtrapped = 1;
                 break;
             }
+            FALLTHROUGH;
             /*FALLTHRU*/
         default:
             if (mptr->mlet == S_GIANT
@@ -2721,6 +2726,7 @@ immune_to_trap(struct monst *mon, unsigned ttype)
         if (pm->msize <= MZ_SMALL
             || amorphous(pm) || is_whirly(pm) || unsolid(pm))
             return TRAP_CLEARLY_IMMUNE;
+        FALLTHROUGH;
         /*FALLTHRU*/
     case SQKY_BOARD:
     case LANDMINE:
@@ -2809,6 +2815,7 @@ immune_to_trap(struct monst *mon, unsigned ttype)
            for monsters, only replicates fire trap, so fall through */
         if (is_you)
             return TRAP_NOT_IMMUNE;
+        FALLTHROUGH;
         /*FALLTHRU*/
     case FIRE_TRAP: /* can always destroy items being carried */
         /* harmful if not resistant or if carrying anything that could burn */
@@ -3244,10 +3251,12 @@ launch_obj(
         /* use otrapped as a flag to ohitmon */
         singleobj->otrapped = 1;
         style &= ~LAUNCH_KNOWN;
+        FALLTHROUGH;
     /*FALLTHRU*/
     case ROLL:
  roll:
         delaycnt = 2;
+        FALLTHROUGH;
     /*FALLTHRU*/
     default:
         if (!delaycnt)
@@ -3352,6 +3361,7 @@ launch_obj(
                     /* if trap doesn't work, skip "disappears" message */
                     if (newlev == depth(&u.uz))
                         break;
+                    FALLTHROUGH;
                     /*FALLTHRU*/
                 case TELEP_TRAP:
                     if (cansee(x, y))
@@ -4050,6 +4060,7 @@ float_down(
         case TRAPDOOR:
             if (!Can_fall_thru(&u.uz) || u.ustuck)
                 break;
+            FALLTHROUGH;
             /*FALLTHRU*/
         default:
             if (!u.utrap) /* not already in the trap */

src/uhitm.c

@@ -3772,6 +3772,7 @@ mhitm_ad_deth(
                 mhm->damage = 0;
                 return;
             }
+            FALLTHROUGH;
             /*FALLTHRU*/
         default: /* case 16: ... case 5: */
             You_feel("your life force draining away...");
@@ -5385,10 +5386,12 @@ hmonas(struct monst *mon)
         case AT_CLAW:
             if (uwep && !cantwield(gy.youmonst.data) && !weapon_used)
                 goto use_weapon;
+            FALLTHROUGH;
             /*FALLTHRU*/
         case AT_TUCH:
             if (uwep && gy.youmonst.data->mlet == S_LICH && !weapon_used)
                 goto use_weapon;
+            FALLTHROUGH;
             /*FALLTHRU*/
         case AT_KICK:
         case AT_BITE:
@@ -5632,6 +5635,7 @@ hmonas(struct monst *mon)
                  || gy.youmonst.data->mlet == S_ORC
                  || gy.youmonst.data->mlet == S_GNOME) && !weapon_used)
                 goto use_weapon;
+            FALLTHROUGH;
             /*FALLTHRU*/
 
         case AT_NONE:
@@ -6016,6 +6020,8 @@ passive_obj(
             }
             break;
         }
+	FALLTHROUGH;
+	/* FALLTHRU */
     default:
         break;
     }

src/weapon.c

@@ -1466,7 +1466,9 @@ weapon_hit_bonus(struct obj *weapon)
     } else if (type <= P_LAST_WEAPON) {
         switch (P_SKILL(type)) {
         default:
-            impossible(bad_skill, P_SKILL(type)); /* fall through */
+            impossible(bad_skill, P_SKILL(type));
+            FALLTHROUGH;
+            /* FALLTHRU */
         case P_ISRESTRICTED:
         case P_UNSKILLED:
             bonus = -4;
@@ -1487,7 +1489,9 @@ weapon_hit_bonus(struct obj *weapon)
             skill = P_SKILL(wep_type);
         switch (skill) {
         default:
-            impossible(bad_skill, skill); /* fall through */
+            impossible(bad_skill, skill);
+            FALLTHROUGH;
+            /* FALLTHRU */
         case P_ISRESTRICTED:
         case P_UNSKILLED:
             bonus = -9;
@@ -1561,7 +1565,8 @@ weapon_dam_bonus(struct obj *weapon)
         switch (P_SKILL(type)) {
         default:
             impossible("weapon_dam_bonus: bad skill %d", P_SKILL(type));
-        /* fall through */
+            FALLTHROUGH;
+        /* FALLTHRU */
         case P_ISRESTRICTED:
         case P_UNSKILLED:
             bonus = -2;

src/wizard.c

@@ -284,8 +284,8 @@ strategy(struct monst *mtmp)
     case 1: /* the wiz is less cautious */
         if (mtmp->data != &mons[PM_WIZARD_OF_YENDOR])
             return (unsigned long) STRAT_HEAL;
-    /* else fall through */
+        FALLTHROUGH;
-
+	/* FALLTHRU */
     case 2:
         dstrat = STRAT_HEAL;
         break;
@@ -399,6 +399,7 @@ tactics(struct monst *mtmp)
                 mtmp->mhp += rnd(8);
                 return 1;
             }
+        FALLTHROUGH;
         /*FALLTHRU*/
 
     case STRAT_NONE: /* harass */

src/wizcmds.c

@@ -1065,6 +1065,7 @@ wiz_intrinsic(void)
                    so needs more than simple incr_itimeout() but we want
                    the pline() issued with that */
                 make_glib((int) newtimeout);
+                FALLTHROUGH;
                 /*FALLTHRU*/
             default:
  def_feedback:

src/zap.c

@@ -177,6 +177,7 @@ bhitm(struct monst *mtmp, struct obj *otmp)
     switch (otyp) {
     case WAN_STRIKING:
         zap_type_text = "wand";
+        FALLTHROUGH;
     /*FALLTHRU*/
     case SPE_FORCE_BOLT:
         reveal_invis = TRUE;
@@ -1096,6 +1097,7 @@ revive(struct obj *corpse, boolean by_hero)
             obfree(corpse, (struct obj *) 0);
             break;
         }
+        FALLTHROUGH;
         /*FALLTHRU*/
     case OBJ_FREE:
     case OBJ_MIGRATING:
@@ -2056,6 +2058,7 @@ stone_to_flesh_obj(struct obj *obj) /* nonnull */
         smell = TRUE;
         break;
     case WEAPON_CLASS: /* crysknife */
+        FALLTHROUGH;
         /*FALLTHRU*/
     default:
         res = 0;
@@ -2869,6 +2872,7 @@ zapyourself(struct obj *obj, boolean ordinary)
     case WAN_LIGHT: /* (broken wand) */
         /* assert( !ordinary ); */
         damage = d(obj->spe, 25);
+        FALLTHROUGH;
         /*FALLTHRU*/
     case EXPENSIVE_CAMERA:
         if (!damage)
@@ -3243,6 +3247,7 @@ zap_updown(struct obj *obj) /* wand or spell, nonnull */
     case WAN_STRIKING:
     case SPE_FORCE_BOLT:
         striking = TRUE;
+        FALLTHROUGH;
         /*FALLTHRU*/
     case WAN_LOCKING:
     case SPE_WIZARD_LOCK:
@@ -4924,6 +4929,7 @@ dobuzz(
                 switch (bounce) {
                 case 0:
                     dx = -dx;
+                    FALLTHROUGH;
                     /*FALLTHRU*/
                 case 1:
                     dy = -dy;
@@ -5256,6 +5262,7 @@ zap_over_floor(
         break;
 
     case ZT_LIGHTNING:
+        FALLTHROUGH;
         /*FALLTHRU*/
     case ZT_ACID:
         if (lev->typ == IRONBARS) {

sys/msdos/video.c

@@ -293,6 +293,7 @@ term_end_attr(int attr)
     switch (attr) {
     case ATR_INVERSE:
         inversed = 0;
+        FALLTHROUGH;
         /*FALLTHRU*/
     case ATR_ULINE:
     case ATR_BOLD:
@@ -341,6 +342,7 @@ term_start_attr(int attr)
         break;
     case ATR_INVERSE:
         inversed = 1;
+        FALLTHROUGH;
         /*FALLTHRU*/
     default:
         g_attribute = iflags.grmode ? attrib_gr_normal : attrib_text_normal;

sys/unix/hints/include/compiler.370

@@ -78,20 +78,29 @@ CXX=g++ -std=gnu++11
 GCCGTEQ9 := $(shell expr `$(CC) -dumpversion | cut -f1 -d.` \>= 9)
 GCCGTEQ11 := $(shell expr `$(CC) -dumpversion | cut -f1 -d.` \>= 11)
 GCCGTEQ12 := $(shell expr `$(CC) -dumpversion | cut -f1 -d.` \>= 12)
+GCCGTEQ14 := $(shell expr `$(CC) -dumpversion | cut -f1 -d.` \>= 14)
 ifeq "$(GCCGTEQ9)" "1"
 # flags present in gcc version greater than or equal to 9 can go here
 CFLAGS+=-Wformat-overflow
 CFLAGS+=-Wmissing-parameter-type
 endif   # GCC greater than or equal to 9
 #ifeq "$(GCCGTEQ11)" "1"
+CFLAGS+=-Wimplicit-fallthrough
 #endif
 #ifeq "$(GCCGTEQ12)" "1"
 #endif
+#ifeq "$(GCCGTEQ14)" "1"
+CFLAGS+=-std=gnu23
+#endif
 # end of gcc-specific
 else   # gcc or clang?
 CXX=clang++ -std=gnu++11
 # clang-specific follows
+CLANGGTEQ12 := $(shell expr `$(CC) -dumpversion | cut -f1 -d.` \>= 12)
 CLANGGTEQ14 := $(shell expr `$(CC) -dumpversion | cut -f1 -d.` \>= 14)
+ifeq "$(CLANGGTEQ12)" "1"
+CFLAGS+=-Wimplicit-fallthrough
+endif
 ifeq "$(CLANGGTEQ14)" "1"
 ifneq "$(VIEWDEPRECATIONS)" "1"
 CFLAGS+=-Wno-deprecated-declarations

sys/unix/hints/include/cross-pre2.370

@@ -142,6 +142,7 @@ MSDOS_TARGET_CFLAGS = -c -O -I../include -I../sys/msdos -I../win/share \
         -Wall -Wextra -Wno-missing-field-initializers -Wreturn-type -Wunused \
         -Wformat -Wswitch -Wshadow -Wwrite-strings \
 	-Wimplicit -Wimplicit-function-declaration -Wimplicit-int \
+	-Wimplicit-fallthrough \
 	-Wmissing-parameter-type -Wold-style-definition -Wstrict-prototypes
 MSDOS_TARGET_CXXFLAGS = -c -O -I../include -I../sys/msdos -I../win/share \
         $(LUAINCL) -DDLB $(PDCURSESDEF) \

sys/vms/vmsunix.c

@@ -241,6 +241,7 @@ vms_define(const char *name, const char *value, int flag)
     switch (flag) {
     case ENV_JOB: /* job logical name */
         tbl_dsc.len = strlen(tbl_dsc.adr = "LNM$JOB");
+        FALLTHROUGH;
     /*FALLTHRU*/
     case ENV_SUP: /* supervisor-mode process logical name */
         result = lib$set_logical(&nam_dsc, &val_dsc, &tbl_dsc);

sys/windows/Makefile.nmake

@@ -1131,10 +1131,18 @@ scall  =
 # 4777 format string requires an argument of type 'type',
 #      but variadic argument 'position' has type 'type'
 # 4820 padding in struct
+# 5262 enable fallthrough warnings that lack [[fallthrough]]
+#
 ctmpflags = $(ctmpflags:-W3=-W4) -wd4100 -wd4244 -wd4245 -wd4310 -wd4706 -w44777 -wd4820
 !IF ($(VSVER) >= 2019)
 ctmpflags = $(ctmpflags) -w44774
 !ENDIF
+!IF ($(VSVER) >= 2022)
+!IF ($(MAKEVERSION) >= 1440338120)
+# warning 5262 became available starting in Visual Studio 2022 version 17.4.
+ctmpflags = $(ctmpflags) -w45262 /std:clatest
+!ENDIF
+!ENDIF
 !ENDIF
 
 #More verbose warning output options below
@@ -2880,6 +2888,8 @@ $(OTTY)sfstruct.o: sfstruct.c $(HACK_H)
 $(OTTY)shk.o: shk.c $(HACK_H)
 $(OTTY)shknam.o: shknam.c $(HACK_H)
 $(OTTY)sit.o: sit.c $(HACK_H) $(INCL)\artifact.h
+	$(Q)$(CC) $(CFLAGS) /EP  $(@B).c > $(OTTY)$(@B).c.preproc
+	$(Q)$(CC) $(CFLAGS) -Fo$@ $(@B).c
 $(OTTY)sounds.o: sounds.c $(HACK_H)
 $(OTTY)sp_lev.o: sp_lev.c $(HACK_H) $(INCL)\sp_lev.h
 $(OTTY)spell.o: spell.c $(HACK_H)

sys/windows/consoletty.c

@@ -1087,6 +1087,7 @@ CtrlHandler(DWORD ctrltype)
     /* case CTRL_C_EVENT: */
     case CTRL_BREAK_EVENT:
         term_clear_screen();
+        FALLTHROUGH;
     case CTRL_CLOSE_EVENT:
     case CTRL_LOGOFF_EVENT:
     case CTRL_SHUTDOWN_EVENT:
@@ -1335,7 +1336,8 @@ xputc_core(int ch)
     case '\n':
         if (console.cursor.Y < console.height - 1)
             console.cursor.Y++;
-    /* fall through */
+        FALLTHROUGH;
+    /* FALLTHRU */
     case '\r':
         console.cursor.X = 1;
         break;
@@ -1879,6 +1881,7 @@ toggle_mouse_support(void)
 #endif /* VIRTUAL_TERMINAL_SEQUENCES */
                 break;
         case 0:
+                FALLTHROUGH;
                 /*FALLTHRU*/
         default:
 #ifndef VIRTUAL_TERMINAL_SEQUENCES

sys/windows/windmain.c

@@ -630,7 +630,7 @@ process_options(int argc, char * argv[])
                 break;
             } else
                 raw_printf("\nUnknown switch: %s", argv[0]);
-        /* FALL THROUGH */
+        FALLTHROUGH;
         case '?':
             nhusage();
             nethack_exit(EXIT_SUCCESS);

util/makedefs.c

@@ -846,7 +846,8 @@ do_grep_control(char *buf)
         break;
     case '!': /* if not ID */
         isif = 0;
-    /* FALLTHROUGH */
+        FALLTHROUGH;
+    /* FALLTHRU */
     case '?': /* if ID */
         if (grep_sp == GREP_STACK_SIZE - 2) {
             Fprintf(stderr, "stack overflow at line %d.", grep_lineno);
@@ -2302,6 +2303,7 @@ do_objs(void)
                 n_glass_gems++;
                 break;
             }
+            FALLTHROUGH;
             /*FALLTHRU*/
         case VENOM_CLASS:
             /* fall-through from gem class is ok; objects[] used to have
@@ -2311,6 +2313,7 @@ do_objs(void)
                so strip the extra "splash of " off to keep same macros */
             if (!strncmp(objnam, "SPLASH_OF_", 10))
                 objnam += 10;
+            FALLTHROUGH;
             /*FALLTHRU*/
         default:
             Fprintf(ofp, "#define\t");

win/Qt/qt_bind.cpp

@@ -271,6 +271,7 @@ void NetHackQtBind::qt_askname()
             // success; handle plname[] verification below prior to returning
             break;
         }
+        FALLTHROUGH;
         /*FALLTHRU*/
     case -2:
         // Quit
@@ -726,6 +727,7 @@ char NetHackQtBind::qt_more()
             switch (ch) {
             case '\0': // hypothetical
                 ch = '\033';
+                FALLTHROUGH;
                 /*FALLTHRU*/
             case ' ':
             case '\n':

win/curses/cursdial.c

@@ -1556,6 +1556,7 @@ menu_get_selections(WINDOW *win, nhmenu *menu, int how)
                     break;
                 }
             }
+            FALLTHROUGH;
             /*FALLTHRU*/
         default:
             if (curletter > 0 && curletter < 256

win/curses/cursinit.c

@@ -114,6 +114,7 @@ curses_create_main_windows(void)
 
     case 3:
         noperminv_borders = TRUE;
+        FALLTHROUGH;
         /*FALLTHRU*/
     case 1:                     /* On */
         borders = TRUE;
@@ -121,6 +122,7 @@ curses_create_main_windows(void)
 
     case 4:
         noperminv_borders = TRUE;
+        FALLTHROUGH;
         /*FALLTHRU*/
     case 2:                     /* Auto */
         borders = (term_cols >= 80 + 2 && term_rows >= 24 + 2);

win/curses/cursmisc.c

@@ -291,7 +291,7 @@ curses_break_str(const char *str, int width, int line_num)
     char *retstr;
     int curline = 0;
     int strsize = (int) strlen(str) + 1;
-#if defined(__STDC_VERSION__) && __STDC_VERSION__ >= 199901L
+#if (defined(__STDC_VERSION__) && __STDC_VERSION__ >= 199901L) && !defined(_MSC_VER)
     char substr[strsize];
     char curstr[strsize];
     char tmpstr[strsize];
@@ -363,7 +363,7 @@ curses_str_remainder(const char *str, int width, int line_num)
     char *retstr;
     int curline = 0;
     int strsize = strlen(str) + 1;
-#if __STDC_VERSION__ >= 199901L
+#if (__STDC_VERSION__ >= 199901L) && !defined(_MSC_VER)
     char substr[strsize];
     char tmpstr[strsize];
 
@@ -801,6 +801,7 @@ curses_convert_keys(int key)
            a value for ^H greater than 255 is passed back to core's
            readchar() and stripping the value down to 0..255 yields ^G! */
         ret = C('H');
+        FALLTHROUGH;
         /*FALLTHRU*/
     default:
         if (modifiers_available)

win/curses/cursstat.c

@@ -415,6 +415,7 @@ draw_horizontal(boolean border)
                         w -= (t - 30); /* '+= strlen()' below will add 't';
                                         * functional result being 'w += 30' */
                 }
+                FALLTHROUGH;
                 /*FALLTHRU*/
             case BL_ALIGN:
             case BL_LEVELDESC:
@@ -1231,6 +1232,7 @@ curs_vert_status_vals(int win_width)
                 if (fld_width < hp_width + 3) /* +3: " " gap and "("...")" */
                     Sprintf(leadingspace, "%*s",
                             (hp_width + 3) - fld_width, " ");
+                FALLTHROUGH;
                 /*FALLTHRU*/
             case BL_VERS:
             case BL_EXP:

win/curses/curswins.c

@@ -98,6 +98,7 @@ curses_create_window(int wid, int width, int height, orient orientation)
     switch (orientation) {
     default:
         impossible("curses_create_window: Bad orientation");
+        FALLTHROUGH;
         /*FALLTHRU*/
     case CENTER:
         startx = (term_cols / 2) - (width / 2);

win/tty/termcap.c

@@ -1341,6 +1341,7 @@ s_atr2str(int n)
         /* if italic isn't available, fall through to underline */
         if (ZH && *ZH)
             return ZH;
+        FALLTHROUGH;
         /*FALLTHRU*/
     case ATR_BLINK:
     case ATR_ULINE:
@@ -1351,6 +1352,7 @@ s_atr2str(int n)
             if (nh_US && *nh_US)
                 return nh_US;
         }
+        FALLTHROUGH;
         /*FALLTHRU*/
     case ATR_BOLD:
         if (MD && *MD)
@@ -1378,15 +1380,18 @@ e_atr2str(int n)
         /* send ZR unless we didn't have ZH and substituted US */
         if (ZR && *ZR && ZH && *ZH)
             return ZR;
+        FALLTHROUGH;
         /*FALLTHRU*/
     case ATR_ULINE:
         if (nh_UE && *nh_UE)
             return nh_UE;
+        FALLTHROUGH;
         /*FALLTHRU*/
     case ATR_BOLD:
     case ATR_BLINK:
         if (nh_HE && *nh_HE)
             return nh_HE;
+        FALLTHROUGH;
         /*FALLTHRU*/
     case ATR_DIM:
     case ATR_INVERSE:

win/tty/wintty.c

@@ -665,6 +665,7 @@ tty_askname(void)
         case -1:
             bail("Until next time then..."); /* quit */
             /*NOTREACHED*/
+            break;
         case 0:
             break; /* no game chosen; start new game */
         case 1:
@@ -1084,6 +1085,7 @@ tty_clear_nhwindow(winid window)
     case NHW_MAP:
         /* cheap -- clear the whole thing and tell nethack to redraw botl */
         disp.botlx = TRUE;
+        FALLTHROUGH;
         /*FALLTHRU*/
     case NHW_BASE:
         /* if erasing_tty_screen is True, calling sequence is
@@ -1721,6 +1723,7 @@ process_menu_window(winid window, struct WinDesc *cw)
             break;
         case MENU_EXPLICIT_CHOICE:
             morc = really_morc;
+            FALLTHROUGH;
         /*FALLTHRU*/
         default:
             if (cw->how == PICK_NONE || !strchr(resp, morc)) {
@@ -1878,12 +1881,14 @@ tty_display_nhwindow(
             tty_display_nhwindow(WIN_MESSAGE, TRUE);
             return;
         }
+        FALLTHROUGH;
         /*FALLTHRU*/
     case NHW_BASE:
         (void) fflush(stdout);
         break;
     case NHW_TEXT:
         cw->maxcol = ttyDisplay->cols; /* force full-screen mode */
+        FALLTHROUGH;
         /*FALLTHRU*/
     case NHW_MENU:
         cw->active = 1;
@@ -1951,6 +1956,7 @@ tty_dismiss_nhwindow(winid window)
         if (ttyDisplay->toplin != TOPLINE_EMPTY)
             tty_display_nhwindow(WIN_MESSAGE, TRUE);
         nhassert(ttyDisplay->toplin == TOPLINE_EMPTY);
+        FALLTHROUGH;
         /*FALLTHRU*/
     case NHW_STATUS:
     case NHW_BASE:
@@ -4438,6 +4444,7 @@ tty_status_update(
     switch (fldidx) {
     case BL_RESET:
         reset_state = FORCE_RESET;
+        FALLTHROUGH;
         /*FALLTHRU*/
     case BL_FLUSH:
         if (make_things_fit(reset_state) || truncation_expected) {
@@ -4458,6 +4465,7 @@ tty_status_update(
         break;
     case BL_GOLD:
         text = decode_mixed(goldbuf, text);
+        FALLTHROUGH;
         /*FALLTHRU*/
     default:
         attrmask = (color >> 8) & 0x00FF;
@@ -4506,6 +4514,7 @@ tty_status_update(
         break;
     case BL_LEVELDESC:
         dlvl_shrinklvl = 0; /* caller is passing full length string */
+        FALLTHROUGH;
         /*FALLTHRU*/
     case BL_HUNGER:
         /* The core sends trailing blanks for some fields.

win/win32/mhdlg.c

@@ -146,7 +146,8 @@ GetlinDlgProc(HWND hWnd, UINT message, WPARAM wParam, LPARAM lParam)
                                (WPARAM) sizeof(wbuf2), (LPARAM) wbuf2);
             NH_W2A(wbuf2, data->result, data->result_size);
 
-        /* Fall through. */
+        FALLTHROUGH;
+        /* FALLTHRU */
 
         /* cancel button was pressed */
         case IDCANCEL:
@@ -246,7 +247,8 @@ ExtCmdDlgProc(HWND hWnd, UINT message, WPARAM wParam, LPARAM lParam)
                 hWnd, IDC_EXTCMD_LIST, LB_GETCURSEL, (WPARAM) 0, (LPARAM) 0);
             if (*data->selection == LB_ERR)
                 *data->selection = -1;
-        /* Fall through. */
+            FALLTHROUGH;
+            /* FALLTHRU */
 
         /* CANCEL button ws clicked */
         case IDCANCEL: