c++: constexpr bit_cast with empty field

The change to only cache constexpr calls that are
reduced_constant_expression_p tripped on bit-cast3.C, which failed that
predicate due to the presence of an empty field in the result of
native_interpret_aggregate, which reduced_constant_expression_p rejects to
avoid confusing output_constructor.

This patch proposes to skip such fields in native_interpret_aggregate, since
they aren't actually involved in the value representation.

gcc/ChangeLog:

	* fold-const.cc (native_interpret_aggregate): Skip empty fields.

gcc/cp/ChangeLog:

	* constexpr.cc (cxx_eval_bit_cast): Check that the result of
	native_interpret_aggregate doesn't need more evaluation.

1 files changed, 9 insertions, 0 deletions

diff --git a/gcc/cp/constexpr.cc b/gcc/cp/constexpr.cc
index 9d85c3b..6e8f1c2 100644
--- a/gcc/cp/constexpr.cc
+++ b/gcc/cp/constexpr.cc
@@ -1440,6 +1440,8 @@ enum value_cat {
 
 static tree cxx_eval_constant_expression (const constexpr_ctx *, tree,
 					  value_cat, bool *, bool *, tree * = NULL);
+static tree cxx_eval_bare_aggregate (const constexpr_ctx *, tree,
+				     value_cat, bool *, bool *);
 static tree cxx_fold_indirect_ref (const constexpr_ctx *, location_t, tree, tree,
 				   bool * = NULL);
 static tree find_heap_var_refs (tree *, int *, void *);
@@ -4803,6 +4805,13 @@ cxx_eval_bit_cast (const constexpr_ctx *ctx, tree t, bool *non_constant_p,
 	{
 	  clear_type_padding_in_mask (TREE_TYPE (t), mask);
 	  clear_uchar_or_std_byte_in_mask (loc, r, mask);
+	  if (CHECKING_P)
+	    {
+	      tree e = cxx_eval_bare_aggregate (ctx, r, vc_prvalue,
+						non_constant_p, overflow_p);
+	      gcc_checking_assert (e == r);
+	      r = e;
+	    }
 	}
     }