c++: dependent bases and 'this' availability [PR103831]

c++: dependent bases and 'this' availability [PR103831]

Here during satisfaction of B's constraints we're failing to reject the
object-less call to the non-static member function A::size ultimately
because satisfaction is performed in the (access) context of the class
template B, which has a dependent base, and so the any_dependent_bases_p
check within build_new_method_call causes us to not reject the call.
(Subsequent constexpr evaluation of the call succeeds since the function
is effectively static.)

This patch fixes this by refining the any_dependent_bases_p check within
build_new_method_call: if we're in a context where 'this' is unavailable,
then we cannot resolve the implicit object regardless of the presence of
a dependent base.  So let's also check current_class_ptr alongside a_d_b_p.

	PR c++/103831

gcc/cp/ChangeLog:

	* call.c (build_new_method_call): Consider dependent bases only
	if 'this' is available.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/concepts-class3.C: New test.
	* g++.dg/template/non-dependent18.C: New test.