[PR c++/84943] mark function as used when taking its address

[PR c++/84943] mark function as used when taking its address

fn[0]() ICEd because we would fold the INDIRECT_REF used for the
array indexing while building the address for the call, after not
finding the decl hiding there at first.  But the decl would be exposed
by the folding, and then lower layers would complain we had the decl,
after all, but it wasn't one of the artificial or special functions
that could be called without being marked as used.

This patch arranges for a FUNCTION_DECL to be marked as used when
taking its address, just like we already did when taking the address
of a static function to call it as a member function (i.e. using the
obj.fn() notation).  However, we shouldn't mark functions as used when
just performing overload resolution, lest we might instantiate
templates we shouldn't, as in g++.dg/overload/template1.C, so we
adjust mark_used to return early when testing conversions.

for  gcc/cp/ChangeLog

	PR c++/84943
	* typeck.c (cp_build_addr_expr_1): Mark FUNCTION_DECL as
	used.
	* decl2.c (mark_used): Return without effects if tf_conv.

for  gcc/testsuite/ChangeLog

	PR c++/84943
	* g++.dg/pr84943.C: New.
	* g++.dg/pr84943-2.C: New.

From-SVN: r259067