(svn r13348) -Fix: an industry does not always need foundations; in the case of NewGRFs an industry can be built on a steep slope, however when 'querying' the foundation type of an industry we neglected the fact that these steep slope tiles do not have a foundation. As a result of this when one built a structure requiring foundations on a tile SW or SE of a steep slope industry tile it would try to apply the foundation on the steep slope to determine whether to draw the wall of the foundation and trigger an assert.

src/industry_cmd.cpp

@@ -334,6 +334,20 @@ static uint GetSlopeZ_Industry(TileIndex tile, uint x, uint y)
 
 static Foundation GetFoundation_Industry(TileIndex tile, Slope tileh)
 {
+	IndustryGfx gfx = GetIndustryGfx(tile);
+
+	/* For NewGRF industry tiles we might not be drawing a foundation. We need to
+	 * account for this, otherwise we might be applying a FOUNDATION_LEVELED
+	 * on a steep slope which is not allowed. Furthermore other structures should
+	 * draw the wall of the foundation in this case.
+	 */
+	if (gfx >= NEW_INDUSTRYTILEOFFSET) {
+		const IndustryTileSpec *indts = GetIndustryTileSpec(gfx);
+		if (indts->grf_prop.spritegroup != NULL && HasBit(indts->callback_flags, CBM_INDT_DRAW_FOUNDATIONS)) {
+			uint32 callback_res = GetIndustryTileCallback(CBID_INDUSTRY_DRAW_FOUNDATIONS, 0, 0, gfx, GetIndustryByTile(tile), tile);
+			if (callback_res == 0) return FOUNDATION_NONE;
+		}
+	}
 	return FlatteningFoundation(tileh);
 }