Fix #8168: allow relocating of HQ partial over existing HQ (#8510)

Just in case you want to move it SLIGHTLY to the right.

regression/regression/main.nut

@@ -426,7 +426,7 @@ function Regression::Company()
 	print("  GetCompanyHQ():                    " + AICompany.GetCompanyHQ(AICompany.COMPANY_SELF));
 	print("  BuildCompanyHQ():                  " + AICompany.BuildCompanyHQ(AIMap.GetTileIndex(129, 129)));
 	print("  GetCompanyHQ():                    " + AICompany.GetCompanyHQ(AICompany.COMPANY_SELF));
-	print("  BuildCompanyHQ():                  " + AICompany.BuildCompanyHQ(AIMap.GetTileIndex(129, 128)));
+	print("  BuildCompanyHQ():                  " + AICompany.BuildCompanyHQ(AIMap.GetTileIndex(239, 76)));
 	print("  GetLastErrorString():              " + AIError.GetLastErrorString());
 	print("  GetAutoRenewStatus();              " + AICompany.GetAutoRenewStatus(AICompany.COMPANY_SELF));
 	print("  SetAutoRenewStatus(true);          " + AICompany.SetAutoRenewStatus(true));

src/object_cmd.cpp

@@ -248,7 +248,14 @@ CommandCost CmdBuildObject(TileIndex tile, DoCommandFlag flags, uint32 p1, uint3
 			} else {
 				if (!allow_ground) return_cmd_error(STR_ERROR_MUST_BE_BUILT_ON_WATER);
 				/* For non-water tiles, we'll have to clear it before building. */
-				cost.AddCost(DoCommand(t, 0, 0, flags & ~DC_EXEC, CMD_LANDSCAPE_CLEAR));
+
+				/* When relocating HQ, allow it to be relocated (partial) on itself. */
+				if (!(type == OBJECT_HQ &&
+						IsTileType(t, MP_OBJECT) &&
+						IsTileOwner(t, _current_company) &&
+						IsObjectType(t, OBJECT_HQ))) {
+					cost.AddCost(DoCommand(t, 0, 0, flags & ~DC_EXEC, CMD_LANDSCAPE_CLEAR));
+				}
 			}
 		}