PR c++/51611
	* cp-tree.h (CONVERT_EXPR_VBASE_PATH): New.
	* class.c (build_base_path): Defer vbase conversion in an NSDMI.
	* tree.c (bot_replace): Expand it here.
	* cp-gimplify.c (cp_genericize_r): Make sure deferred conversion
	doesn't leak into GENERIC.

From-SVN: r182602