I forgot to add to src/script/api/ai_changelog.hpp and src/script/api/game_changelog.hpp. I'll fix that.

ScriptTile::IsWaterTile seems to return true for sea, canal and river.
The new ScriptTile::IsRiverTile return true for river.

What is so special about rivers, that they need a specific check, while sea and canal do not?

Same question holds for GetBuildCost()

Maybe an API change - GetWaterTile that returns none, sea, canal or river ?

I don't know the reasoning why you need only this but not IsCanalTile. Both river and canal share the property that it can be deleted. For a path finder it may be useful to get to know of both canal and river tiles if you like to destroy them. If you want to bridge, then the IsWaterTile should probably be sufficient.

The PR is based on a patch that seemed valid, but I don't know all the details of the use case that caused it to be written.

Apparently there is ScriptMarine::IsCanalTile.

Suggestion:

• Add ScriptTile::IsRiverTile, ScriptTile::IsSeaTile
• Add ScriptTile.BT_CLEAR_RIVER, ScriptTile.BT_CLEAR_SEA

Ideally the docs should link between them :)

Closed until there is an usecase.

It's still possible to properly detect a river tile with the current API, but it required me to read OpenTTD source to understand how the functions do their things.

function IsRiverTile(tile)
{
    if (!AITile.IsWaterTile(tile)) return false;
    if (AITile.GetMaxHeight(tile) == 0) return false;
    if (AIMarine.IsWaterDepotTile(tile)) return false;
    if (AIMarine.IsCanalTile(tile)) return false;
    if (AIMarine.IsLockTile(tile)) return false;
    return true;
}

@SamuXarick Looking at the code above, it looks like your version of ScriptTile.IsRiverTile may return a false negative for rivers that are on tile height 0 (possible in the scenario editor) as well as rivers that have locks or water depots (both currently possible ingame).

Oops, I wasn't aware it was possible to have rivers on tile height 0 via scenario editor.

Rivers that have locks or water depots don't count. The submitted code was also not considering them.

/* static */ bool ScriptTile::IsRiverTile(TileIndex tile)
{
	if (!::IsValidTile(tile)) return false;
	return ::IsTileType(tile, MP_WATER) && ::IsRiver(tile);
}

So, it seems that there isn't a way to detect river tiles in all case scenarios. I can't think of a workaround atm.