c++: constexpr ref to array of array [PR102307]

c++: constexpr ref to array of array [PR102307]

The problem here is that first check_initializer calls
build_aggr_init_full_exprs, which does overload resolution, but then in the
case of failed constexpr throws away the result and does it again in
build_functional_cast.  But in the first overload resolution,
reshape_init_array_1 decided to reuse the inner CONSTRUCTORs because
tf_error is set, so we know we're committed.  But the second pass gets
confused by the CONSTRUCTORs with non-init-list types.

Fixed by avoiding a second pass: instead, pass the call from build_aggr_init
to build_cplus_new, which will turn it into a TARGET_EXPR.  I don't bother
to change the object argument because it will be replaced later in
simplify_aggr_init_expr.

	PR c++/102307

gcc/cp/ChangeLog:

	* decl.c (check_initializer): Use build_cplus_new in case of
	constexpr failure.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp1z/constexpr-array2.C: New test.