c++: Fix up ICE on __builtin_shufflevector constexpr evaluation [PR105871]
As the following testcase shows, BIT_FIELD_REF result doesn't have to have
just integral type, it can also have vector type. And in that case
cxx_eval_bit_field_ref just ICEs on it because it is unprepared for that
case, creates the initial value with build_int_cst (sure, that one could be
easily replaced with build_zero_cst) and then expects it can through shifts,
ands and ors come up with the final value, but that doesn't work for
vectors.
We already call fold_ternary if whole is a VECTOR_CST, this patch does the
same if the result doesn't have integral type. And, there is no guarantee
fold_ternary will succeed and the callers certainly don't expect NULL
being returned, so it also diagnoses those as non-constant and returns
original t in that case.
2022-06-09 Jakub Jelinek <jakub@redhat.com>
PR c++/105871
* constexpr.cc (cxx_eval_bit_field_ref): For BIT_FIELD_REF with
non-integral result type use fold_ternary too like for BIT_FIELD_REFs
from VECTOR_CST. If fold_ternary returns NULL, diagnose non-constant
expression, set *non_constant_p and return t, instead of returning
NULL.