In the example of the patch, g++ fails to warn that the variable N::i
(introduced via a using declaration) is unused.

This is because as we want to emit the warning in poplevel, when we
walk the local bindings returned by getdecls, we forget that a
VAR_DECL introduced by a using declaration is represented by a
TREE_LIST which TREE_VALUE is the VAR_DECL, and we wrongly look for a
bare VAR_DECL.

Fixed thus and tested on x86_64-unknown-linux-gnu against trunk.

gcc/cp/

	* decl.c (poplevel<warn_unused*>): Do not forget that some local
	bindings are represented by a TREE_LIST.

gcc/testsuite/

	* g++.dg/warn/Wunused-var-18.C: New test.

From-SVN: r191829