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Ie., the player would be able to toss a grenade to the asterisk.
I fixed this by drawing a line (using the Bresenham algorithm) to the target square to make sure there's a clear path. Monsters don't yet interfere with the throw (I guess the player is logging the item) but that's something I need to look at.
Also, the larger issue is that there are two different methods in the game for the throwing and shooting. When you use a grenade or a flare you pick your target square as described, when you fire a gun you must pick a direction, with the consequence that you must fire along a cardinal direction. I kind of like that, because it forces the some tactical maneuvering, but it seems kind of ugly having the two different methods.
Perhaps at some point I'll switch to DoomRL style targeting, though.