re-enable -Wunreachable-code under clang

whitelist the valid cases showing up

If an earlier version of clang is showing more cases (particularly
if they don't make sense), the re-enabling of the warning in
sys/unix/hints/include/compiler.2020 can be made clang-version
specific instead. I had no way to test earlier versions.

src/do_name.c

@@ -530,11 +530,14 @@ coord_desc(int x, int y, char *outbuf, char cmode)
         /* for normal map sizes, force a fixed-width formatting so that
            /m, /M, /o, and /O output lines up cleanly; map sizes bigger
            than Nx999 or 999xM will still work, but not line up like normal
-           when displayed in a column setting */
+           when displayed in a column setting.
+
+	   The (100) is placed in brackets below to mark the [: "03"] as
+           explicit compile-time dead code for clang */
         if (!*screen_fmt)
             Sprintf(screen_fmt, "[%%%sd,%%%sd]",
-                    (ROWNO - 1 + 2 < 100) ? "02" :  "03",
-                    (COLNO - 1 < 100) ? "02" : "03");
+                    (ROWNO - 1 + 2 < (100)) ? "02" :  "03",
+                    (COLNO - 1 < (100)) ? "02" : "03");
         /* map line 0 is screen row 2;
            map column 0 isn't used, map column 1 is screen column 1 */
         Sprintf(outbuf, screen_fmt, y + 2, x);

src/files.c

@@ -923,7 +923,10 @@ set_savefile_name(boolean regularize_it)
             overflow = 2;
     }
 #ifdef SAVE_EXTENSION
-    if (strlen(SAVE_EXTENSION) > 0 && !overflow) {
+    /* (0) is placed in brackets below so that the [&& !overflow] is
+       explicit dead code (the ">" comparison is detected as always
+       FALSE at compile-time). Done to appease clang's -Wunreachable-code */
+    if (strlen(SAVE_EXTENSION) > (0) && !overflow) {
         if (strlen(g.SAVEF) + strlen(SAVE_EXTENSION) < (SAVESIZE - 1)) {
             Strcat(g.SAVEF, SAVE_EXTENSION);
 #ifdef MSDOS

src/mthrowu.c

@@ -579,10 +579,9 @@ m_throw(
                 potionhit(&g.youmonst, singleobj, POTHIT_MONST_THROW);
                 break;
             }
-
             oldumort = u.umortality;
+
             switch (singleobj->otyp) {
-                int dam, hitv;
             case EGG:
                 if (!touch_petrifies(&mons[singleobj->corpsenm])) {
                     impossible("monster throwing egg type %d",
@@ -596,24 +595,28 @@ m_throw(
                 hitu = thitu(8, 0, &singleobj, (char *) 0);
                 break;
             default:
-                dam = dmgval(singleobj, &g.youmonst);
-                hitv = 3 - distmin(u.ux, u.uy, mon->mx, mon->my);
-                if (hitv < -4)
-                    hitv = -4;
-                if (is_elf(mon->data)
-                    && objects[singleobj->otyp].oc_skill == P_BOW) {
-                    hitv++;
-                    if (MON_WEP(mon) && MON_WEP(mon)->otyp == ELVEN_BOW)
+                {
+                    int dam, hitv;
+
+                    dam = dmgval(singleobj, &g.youmonst);
+                    hitv = 3 - distmin(u.ux, u.uy, mon->mx, mon->my);
+                    if (hitv < -4)
+                        hitv = -4;
+                    if (is_elf(mon->data)
+                        && objects[singleobj->otyp].oc_skill == P_BOW) {
                         hitv++;
-                    if (singleobj->otyp == ELVEN_ARROW)
-                        dam++;
+                        if (MON_WEP(mon) && MON_WEP(mon)->otyp == ELVEN_BOW)
+                            hitv++;
+                        if (singleobj->otyp == ELVEN_ARROW)
+                            dam++;
+                    }
+                    if (bigmonst(g.youmonst.data))
+                        hitv++;
+                    hitv += 8 + singleobj->spe;
+                    if (dam < 1)
+                        dam = 1;
+                    hitu = thitu(hitv, dam, &singleobj, (char *) 0);
                 }
-                if (bigmonst(g.youmonst.data))
-                    hitv++;
-                hitv += 8 + singleobj->spe;
-                if (dam < 1)
-                    dam = 1;
-                hitu = thitu(hitv, dam, &singleobj, (char *) 0);
             }
             if (hitu && singleobj->opoisoned && is_poisonable(singleobj)) {
                 char onmbuf[BUFSZ], knmbuf[BUFSZ];

src/nhlua.c

@@ -116,6 +116,8 @@ l_nhcore_call(int callidx)
     }
 }
 
+DISABLE_WARNING_UNREACHABLE_CODE
+
 void
 nhl_error(lua_State *L, const char *msg)
 {
@@ -136,8 +138,11 @@ nhl_error(lua_State *L, const char *msg)
 #endif
     (void) lua_error(L);
     /*NOTREACHED*/
+    /* UNREACHABLE_CODE */
 }
 
+RESTORE_WARNING_UNREACHABLE_CODE
+
 /* Check that parameters are nothing but single table,
    or if no parameters given, put empty table there */
 void

src/sp_lev.c

@@ -1570,6 +1570,8 @@ create_subroom(
     return TRUE;
 }
 
+DISABLE_WARNING_UNREACHABLE_CODE
+
 /*
  * Create a new door in a room.
  * It's placed on a wall (north, south, east or west).
@@ -1659,6 +1661,7 @@ create_door(room_door* dd, struct mkroom* broom)
         default:
             x = y = 0;
             panic("create_door: No wall for door!");
+            /*UNREACHABLE_CODE*/
             goto outdirloop;
         }
  outdirloop:
@@ -1675,6 +1678,8 @@ create_door(room_door* dd, struct mkroom* broom)
     levl[x][y].doormask = dd->mask;
 }
 
+RESTORE_WARNING_UNREACHABLE_CODE
+
 /*
  * Create a secret door in croom on any one of the specified walls.
  */