I've been thinking for a while that the 'lval' parameter needed a third
value for discarded-value expressions; most importantly,
cxx_eval_store_expression does extra work for an lvalue result, and we also
don't want to do the l->r conversion.
Mostly this is pretty mechanical. Apart from the _store_ fix, I also use
vc_discard for substatements of a STATEMENT_LIST other than a stmt-expr
result, and avoid building _REFs to be ignored in a few other places.
gcc/cp/ChangeLog:
* constexpr.cc (enum value_cat): New. Change all 'lval' parameters
from int to value_cat. Change most false to vc_prvalue, most true
to vc_glvalue, cases where the return value is ignored to
vc_discard.
(cxx_eval_statement_list): Only vc_prvalue for stmt-expr result.
(cxx_eval_store_expression): Only build _REF for vc_glvalue.
(cxx_eval_array_reference, cxx_eval_component_reference)
(cxx_eval_indirect_ref, cxx_eval_constant_expression): Likewise.