c++: ICE when building builtin operator->* set [PR103455]

c++: ICE when building builtin operator->* set [PR103455]

Here when constructing the builtin operator->* candidate set according
to the available conversion functions for the operand types, we end up
considering a candidate with C1=T (through B's dependent conversion
function) and C2=F, during which we crash from DERIVED_FROM_P because
dependent_type_p sees a TEMPLATE_TYPE_PARM outside of a template
context.

Sidestepping the question of whether we should be considering such a
dependent conversion function here in the first place, it seems futile
to test DERIVED_FROM_P for anything other than an actual class type, so
this patch fixes this ICE by simply guarding the DERIVED_FROM_P test
with CLASS_TYPE_P instead of MAYBE_CLASS_TYPE_P.

	PR c++/103455

gcc/cp/ChangeLog:

	* call.cc (add_builtin_candidate) <case MEMBER_REF>: Test
	CLASS_TYPE_P instead of MAYBE_CLASS_TYPE_P.

gcc/testsuite/ChangeLog:

	* g++.dg/overload/builtin6.C: New test.