re PR c++/65896 (Erroneous uninitialized variable access error in constexpr…

re PR c++/65896 (Erroneous uninitialized variable access error in constexpr function with temporary variables)

	PR c++/65896
	* constexpr.c (cxx_eval_store_expression): Don't try to actually
	store an empty class.

From-SVN: r222549

gcc/cp/ChangeLog

 2015-04-28  Jason Merrill  <jason@redhat.com>
 
+	PR c++/65896
+	* constexpr.c (cxx_eval_store_expression): Don't try to actually
+	store an empty class.
+
 	PR c++/65656
 	* constexpr.c (cxx_eval_builtin_function_call): Fix
 	__builtin_constant_p.

gcc/cp/constexpr.c

@@ -2580,12 +2580,25 @@ cxx_eval_store_expression (const constexpr_ctx *ctx, tree t,
 
   /* First we figure out where we're storing to.  */
   tree target = TREE_OPERAND (t, 0);
+  tree type = TREE_TYPE (target);
   target = cxx_eval_constant_expression (ctx, target,
 					 true,
 					 non_constant_p, overflow_p);
   if (*non_constant_p)
     return t;
 
+  if (!same_type_ignoring_top_level_qualifiers_p (TREE_TYPE (target), type))
+    {
+      /* For initialization of an empty base, the original target will be
+         *(base*)this, which the above evaluation resolves to the object
+	 argument, which has the derived type rather than the base type.  In
+	 this situation, just evaluate the initializer and return, since
+	 there's no actual data to store.  */
+      gcc_assert (is_empty_class (type));
+      return cxx_eval_constant_expression (ctx, init, false,
+					   non_constant_p, overflow_p);
+    }
+
   /* And then find the underlying variable.  */
   vec<tree,va_gc> *refs = make_tree_vector();
   tree object = NULL_TREE;
@@ -2622,7 +2635,7 @@ cxx_eval_store_expression (const constexpr_ctx *ctx, tree t,
       *non_constant_p = true;
       return t;
     }
-  tree type = TREE_TYPE (object);
+  type = TREE_TYPE (object);
   while (!refs->is_empty())
     {
       if (*valp == NULL_TREE)

gcc/testsuite/g++.dg/cpp0x/constexpr-empty9.C

+// PR c++/65896
+// { dg-do compile { target c++11 } }
+
+struct base {};
+
+struct derived :  base {
+	constexpr derived():
+		base{},
+		m_value(0) {
+	}
+	int m_value;
+};
+
+constexpr int by_ref(derived && value) {
+	return value.m_value;
+}
+
+constexpr int value = by_ref(derived{});