fix #K3890 - "wall_angle: unknown hwall mode 4"

When lastseentyp[x][y] is different from levl[x][y].typ and #terrain
tries to show <x,y>, the value of levl[x][y].flags might not have an
approrpriate value for the remembered terrain type.  The reported
problem was an impossible() about wall mode.

rm.wall_info == 4 corresponds to rm.doormask == D_OPEN and both of
them are overloaded on rm.flags.  A spot remembered as a wall but
actually a secret door might cause this if it has become discovered
('wall' changed to closed door) and then opened (with door intact)
while out of view.  [I'm not sure how that could happen though.]

I was unable to reproduce it so haven't verified that the fix works.

doc/fixes3-7-0.txt

@@ -1159,6 +1159,9 @@ change helm of brilliance from iron to crystal so that it no longer needs to
 fake player monsters in endgame had a 1 in 1000 chance to be given a stack
 	of 0 gold pieces, eventually triggering an impossible warning (cited
 	one was "Calculating weight of 0 gold pieces?")
+avoid "wall_angle: unknown hwall mode 4" when using #terrain command to
+	display a spot remembered as a wall but actually a secret door that
+	has been transformed into an open door while not in sight
 
 
 Fixes to 3.7.0-x General Problems Exposed Via git Repository

src/detect.c

@@ -2061,18 +2061,26 @@ reveal_terrain_getglyph(coordxy x, coordxy y, int full, unsigned swallowed,
                     && M_AP_TYPE(mtmp) == M_AP_FURNITURE) {
                     glyph = cmap_to_glyph(mtmp->mappearance);
                 } else {
-                    /* we have a topology type but we want a screen
+                    struct rm save_spot;
-                       symbol in order to derive a glyph; some screen
-                       symbols need the flags field of levl[][] in
-                       addition to the type (to disambiguate STAIRS to
-                       S_upstair or S_dnstair, for example; current
-                       flags might not be intended for remembered type,
-                       but we've got no other choice) */
-                    schar save_typ = levl[x][y].typ;
 
+                    /*
+                     * We have a topology type but we want a screen symbol
+                     * in order to derive a glyph.  Some screen symbols need
+                     * the flags field of levl[][] in addition to the type
+                     * (to disambiguate STAIRS to S_upstair or S_dnstair,
+                     * for example).  Current flags might not be intended
+                     * for remembered type, but we've got no other choice.
+                     * An exception is wall_info which can be recalculated and
+                     * needs to be.  Otherwise back_to_glyph() -> wall_angle()
+                     * might issue an impossible() for it if it is currently
+                     * doormask==D_OPEN for an open door remembered as a wall.
+                     */
+                    save_spot = levl[x][y];
                     levl[x][y].typ = gl.lastseentyp[x][y];
+                    if (IS_WALL(levl[x][y].typ))
+                        xy_set_wall_state(x, y); /* levl[x][y].wall_info */
                     glyph = back_to_glyph(x, y);
-                    levl[x][y].typ = save_typ;
+                    levl[x][y] = save_spot;
                 }
             }
         }