c++: cxx_eval_array_reference and empty elem type [PR101194]

Here the initializer for x is represented as an empty CONSTRUCTOR due to
its empty element type.  So during constexpr evaluation of the ARRAY_REF
x[0], we end up trying to value initialize the omitted element at index 0,
which fails because the element type is not default constructible.

This patch makes cxx_eval_array_reference specifically handle the case
where the element type is an empty type.

	PR c++/101194

gcc/cp/ChangeLog:

	* constexpr.c (cxx_eval_array_reference): When the element type
	is an empty type and the corresponding element is omitted, just
	return an empty CONSTRUCTOR instead of attempting value
	initialization.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/constexpr-empty16.C: New test.

2 files changed, 13 insertions, 1 deletions

diff --git a/gcc/cp/constexpr.c b/gcc/cp/constexpr.c
index 4cd9db3..39787f3 100644
--- a/gcc/cp/constexpr.c
+++ b/gcc/cp/constexpr.c
@@ -3845,7 +3845,9 @@ cxx_eval_array_reference (const constexpr_ctx *ctx, tree t,
      directly for non-aggregates to avoid creating a garbage CONSTRUCTOR.  */
   tree val;
   constexpr_ctx new_ctx;
-  if (CP_AGGREGATE_TYPE_P (elem_type))
+  if (is_really_empty_class (elem_type, /*ignore_vptr*/false))
+    return build_constructor (elem_type, NULL);
+  else if (CP_AGGREGATE_TYPE_P (elem_type))
     {
       tree empty_ctor = build_constructor (init_list_type_node, NULL);
       val = digest_init (elem_type, empty_ctor, tf_warning_or_error);
diff --git a/gcc/testsuite/g++.dg/cpp0x/constexpr-empty16.C b/gcc/testsuite/g++.dg/cpp0x/constexpr-empty16.C
new file mode 100644
index 0000000..79be244
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/constexpr-empty16.C
@@ -0,0 +1,10 @@
+// PR c++/101194
+// { dg-do compile { target c++11 } }
+
+struct nodefault {
+  constexpr nodefault(int) { }
+};
+
+constexpr nodefault x[1] = { nodefault{0} };
+
+constexpr nodefault y = x[0];