re PR c++/50158 (invalid 'variable set but not used' warning (boolean used as an index to an array))

	PR c++/50158
	* typeck.c (cp_build_modify_expr): Call mark_rvalue_use on rhs
	if it has side-effects and needs to be preevaluated.

	* g++.dg/warn/Wunused-var-16.C: New test.

From-SVN: r177992