c++: constexpr array reference and value-initialization [PR101371]

c++: constexpr array reference and value-initialization [PR101371]

This PR gave me a hard time: I saw multiple issues starting with
different revisions.  But ultimately the root cause seems to be
the following, and the attached patch fixes all issues I've found
here.

In cxx_eval_array_reference we create a new constexpr context for the
CP_AGGREGATE_TYPE_P case, but we also have to create it for the
non-aggregate case.  In this test, we are evaluating

  ((B *)this)->a = rhs->a

which means that we set ctx.object to ((B *)this)->a.  Then we proceed
to evaluate the initializer, rhs->a.  For *rhs, we eval rhs, a PARM_DECL,
for which we have (const B &) &c.arr[0] in the hash table.  Then
cxx_fold_indirect_ref gives us c.arr[0].  c is evaluated to {.arr={}} so
c.arr is {}.  Now we want c.arr[0], so we end up in cxx_eval_array_reference
and since we're initializing from {}, we call build_value_init which
gives us an AGGR_INIT_EXPR that calls 'constexpr B::B()'.  Then we
evaluate this AGGR_INIT_EXPR and since its first argument is dummy,
we take ctx.object instead.  But that is the wrong object, we're not
initializing ((B *)this)->a here.  And so we wound up with an
initializer for A, and then crash in cxx_eval_component_reference:

  gcc_assert (DECL_CONTEXT (part) == TYPE_MAIN_VARIANT (TREE_TYPE (whole)));

where DECL_CONTEXT (part) is B (as it should be) but the type of whole
was A.

So create a new object, if there already was one, and the element type
is not a scalar.

	PR c++/101371

gcc/cp/ChangeLog:

	* constexpr.c (cxx_eval_array_reference): Create a new .object
	and .ctor for the non-aggregate non-scalar case too when
	value-initializing.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp1y/constexpr-101371-2.C: New test.
	* g++.dg/cpp1y/constexpr-101371.C: New test.