diff --git a/gcc/cp/ChangeLog b/gcc/cp/ChangeLog
index b506f4bd055..f9246e56b74 100644
--- a/gcc/cp/ChangeLog
+++ b/gcc/cp/ChangeLog
@@ -1,5 +1,9 @@
 2012-02-07  Jason Merrill  <jason@redhat.com>
 
+	PR c++/51675
+	* semantics.c (cx_check_missing_mem_inits): Handle unions.
+	Fix constexpr default constructor logic.
+
 	PR c++/52035
 	* pt.c (tsubst): Strip uninstantiated typedef.
 
diff --git a/gcc/cp/semantics.c b/gcc/cp/semantics.c
index 90199624876..5646fa74717 100644
--- a/gcc/cp/semantics.c
+++ b/gcc/cp/semantics.c
@@ -6025,13 +6025,28 @@ cx_check_missing_mem_inits (tree fun, tree body, bool complain)
   bool bad;
   tree field;
   unsigned i, nelts;
+  tree ctype;
 
   if (TREE_CODE (body) != CONSTRUCTOR)
     return false;
 
-  bad = false;
   nelts = CONSTRUCTOR_NELTS (body);
-  field = TYPE_FIELDS (DECL_CONTEXT (fun));
+  ctype = DECL_CONTEXT (fun);
+  field = TYPE_FIELDS (ctype);
+
+  if (TREE_CODE (ctype) == UNION_TYPE)
+    {
+      if (nelts == 0 && next_initializable_field (field))
+	{
+	  if (complain)
+	    error ("%<constexpr%> constructor for union %qT must "
+		   "initialize exactly one non-static data member", ctype);
+	  return true;
+	}
+      return false;
+    }
+
+  bad = false;
   for (i = 0; i <= nelts; ++i)
     {
       tree index;
@@ -6050,8 +6065,6 @@ cx_check_missing_mem_inits (tree fun, tree body, bool complain)
 	  if (TREE_CODE (field) != FIELD_DECL
 	      || (DECL_C_BIT_FIELD (field) && !DECL_NAME (field)))
 	    continue;
-	  if (!complain)
-	    return true;
 	  ftype = strip_array_types (TREE_TYPE (field));
 	  if (type_has_constexpr_default_constructor (ftype))
 	    {
@@ -6062,6 +6075,8 @@ cx_check_missing_mem_inits (tree fun, tree body, bool complain)
 				   || errorcount != 0);
 	      continue;
 	    }
+	  if (!complain)
+	    return true;
 	  error ("uninitialized member %qD in %<constexpr%> constructor",
 		 field);
 	  bad = true;
diff --git a/gcc/testsuite/ChangeLog b/gcc/testsuite/ChangeLog
index 9f50b24ab3f..482b4899dd1 100644
--- a/gcc/testsuite/ChangeLog
+++ b/gcc/testsuite/ChangeLog
@@ -1,5 +1,8 @@
 2012-02-07  Jason Merrill  <jason@redhat.com>
 
+	PR c++/51675
+	* g++.dg/cpp0x/constexpr-union3.C: New.
+
 	PR c++/52035
 	* g++.dg/lto/pr52035_0.C: New.
 
diff --git a/gcc/testsuite/g++.dg/cpp0x/constexpr-union3.C b/gcc/testsuite/g++.dg/cpp0x/constexpr-union3.C
new file mode 100644
index 00000000000..bac9cabfe53
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/constexpr-union3.C
@@ -0,0 +1,12 @@
+// PR c++/51675
+// { dg-options -std=c++11 }
+
+union foo
+{
+  int x;
+  short y;
+
+  constexpr foo(): x(0) { }
+};
+
+constexpr foo f;