gcc-you can use this code to learn something about gcc, and inquire further into linux,

				// Build don't link:				// 				// Copyright (C) 2000 Free Software Foundation, Inc.				// Contributed by Nathan Sidwell 14 Aug 2000 								// We failed to diagnose when a class friend declaration did not use an				// elaborated type specifier.								struct Y;				struct Z;				struct X				{				  friend class Z;				  friend Y;         // ERROR - friend must use aggr tag				};