c++: constexpr empty subobject elision [PR110197]

Now that init_subob_ctx no longer sets new_ctx.ctor for a subobject of
empty type, it seems we need to ensure its callers also consistently
omit entries in the parent ctx->ctor for such subobjects.  We also need
to allow cxx_eval_array_reference to synthesize an empty subobject even
if the array CONSTRUCTOR has CONSTRUCTOR_NO_CLEARING set.

	PR c++/110197

gcc/cp/ChangeLog:

	* constexpr.cc (cxx_eval_array_reference): Allow synthesizing an
	empty subobject even if CONSTRUCTOR_NO_CLEARING is set.
	(cxx_eval_bare_aggregate): Set 'no_slot' to true more generally
	whenever new_ctx.ctor is set to NULL_TREE by init_subob_ctx,
	i.e. whenever initializing an subobject of empty type.
	(cxx_eval_vec_init_1): Define 'no_slot' as above and use it
	accordingly.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/constexpr-empty18.C: New test.
	* g++.dg/cpp0x/constexpr-empty19.C: New test.

3 files changed, 35 insertions, 7 deletions

diff --git a/gcc/cp/constexpr.cc b/gcc/cp/constexpr.cc
index f2fcb54..da2c311 100644
--- a/gcc/cp/constexpr.cc
+++ b/gcc/cp/constexpr.cc
@@ -4297,6 +4297,9 @@ cxx_eval_array_reference (const constexpr_ctx *ctx, tree t,
 
   /* Not found.  */
 
+  if (is_really_empty_class (elem_type, /*ignore_vptr*/false))
+    return build_constructor (elem_type, NULL);
+
   if (TREE_CODE (ary) == CONSTRUCTOR
       && CONSTRUCTOR_NO_CLEARING (ary))
     {
@@ -4314,9 +4317,7 @@ 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 (is_really_empty_class (elem_type, /*ignore_vptr*/false))
-    return build_constructor (elem_type, NULL);
-  else if (CP_AGGREGATE_TYPE_P (elem_type))
+  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);
@@ -5095,9 +5096,9 @@ cxx_eval_bare_aggregate (const constexpr_ctx *ctx, tree t,
   FOR_EACH_CONSTRUCTOR_ELT (v, i, index, value)
     {
       tree orig_value = value;
-      /* Like in cxx_eval_store_expression, omit entries for empty fields.  */
-      bool no_slot = TREE_CODE (type) == RECORD_TYPE && is_empty_field (index);
       init_subob_ctx (ctx, new_ctx, index, value);
+      /* Like in cxx_eval_store_expression, omit entries for empty fields.  */
+      bool no_slot = new_ctx.ctor == NULL_TREE;
       int pos_hint = -1;
       if (new_ctx.ctor != ctx->ctor && !no_slot)
 	{
@@ -5261,7 +5262,8 @@ cxx_eval_vec_init_1 (const constexpr_ctx *ctx, tree atype, tree init,
       bool reuse = false;
       constexpr_ctx new_ctx;
       init_subob_ctx (ctx, new_ctx, idx, pre_init ? init : elttype);
-      if (new_ctx.ctor != ctx->ctor)
+      bool no_slot = new_ctx.ctor == NULL_TREE;
+      if (new_ctx.ctor != ctx->ctor && !no_slot)
 	{
 	  if (zeroed_out)
 	    CONSTRUCTOR_NO_CLEARING (new_ctx.ctor) = false;
@@ -5306,7 +5308,14 @@ cxx_eval_vec_init_1 (const constexpr_ctx *ctx, tree atype, tree init,
 	}
       if (*non_constant_p)
 	break;
-      if (new_ctx.ctor != ctx->ctor)
+      if (no_slot)
+	{
+	  /* This is an initializer for an empty subobject; now that we've
+	     checked that it's constant, we can ignore it.  */
+	  gcc_checking_assert (i == 0);
+	  break;
+	}
+      else if (new_ctx.ctor != ctx->ctor)
 	{
 	  /* We appended this element above; update the value.  */
 	  gcc_assert ((*p)->last().index == idx);
diff --git a/gcc/testsuite/g++.dg/cpp0x/constexpr-empty18.C b/gcc/testsuite/g++.dg/cpp0x/constexpr-empty18.C
new file mode 100644
index 0000000..4bb9e3d
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/constexpr-empty18.C
@@ -0,0 +1,7 @@
+// PR c++/110197
+// { dg-do compile { target c++11 } }
+
+struct A { constexpr A(int) { } };
+struct B { A a; };
+constexpr B f(int n) { return B{A{n}}; }
+constexpr B b = f(1);
diff --git a/gcc/testsuite/g++.dg/cpp0x/constexpr-empty19.C b/gcc/testsuite/g++.dg/cpp0x/constexpr-empty19.C
new file mode 100644
index 0000000..5ad6768
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/constexpr-empty19.C
@@ -0,0 +1,12 @@
+// PR c++/110197
+// { dg-do compile { target c++11 } }
+
+struct A {
+  constexpr A() : A(__builtin_is_constant_evaluated()) { }
+  constexpr A(int) { }
+};
+constexpr A a1[1] = {{}};
+constexpr A a2[2] = {{}, {}};
+constexpr A a3[3] = {{}, {}, {}};
+constexpr A a4[4] = {};
+constexpr A a5[5] = {};