c++: Fix up default initialization with consteval default ctor [PR96994]

c++: Fix up default initialization with consteval default ctor [PR96994]

> > The following testcase is miscompiled (in particular the a and i
> > initialization).  The problem is that build_special_member_call due to
> > the immediate constructors (but not evaluated in constant expression mode)
> > doesn't create a CALL_EXPR, but returns a TARGET_EXPR with CONSTRUCTOR
> > as the initializer for it,
>
> That seems like the bug; at the end of build_over_call, after you
>
> >        call = cxx_constant_value (call, obj_arg);
>
> You need to build an INIT_EXPR if obj_arg isn't a dummy.

That works.  obj_arg is NULL if it is a dummy from the earlier code.

2020-10-01  Jakub Jelinek  <jakub@redhat.com>

	PR c++/96994
	* call.c (build_over_call): If obj_arg is non-NULL, return INIT_EXPR
	setting obj_arg to call.

	* g++.dg/cpp2a/consteval18.C: New test.