A partially initialized variable isn't constant.

	* constexpr.c (reduced_constant_expression_p): If
	CONSTRUCTOR_NO_IMPLICIT_ZERO, check that all fields are initialized.

From-SVN: r251948

gcc/cp/ChangeLog

+2017-09-09  Jason Merrill  <jason@redhat.com>
+
+	* constexpr.c (reduced_constant_expression_p): If
+	CONSTRUCTOR_NO_IMPLICIT_ZERO, check that all fields are initialized.
+
 2017-09-09  Eric Botcazou  <ebotcazou@adacore.com>
 
 	PR bootstrap/81926

gcc/cp/constexpr.c

@@ -1732,15 +1732,30 @@ reduced_constant_expression_p (tree t)
 
     case CONSTRUCTOR:
       /* And we need to handle PTRMEM_CST wrapped in a CONSTRUCTOR.  */
-      tree elt; unsigned HOST_WIDE_INT idx;
-      FOR_EACH_CONSTRUCTOR_VALUE (CONSTRUCTOR_ELTS (t), idx, elt)
+      tree idx, val, field; unsigned HOST_WIDE_INT i;
+      if (CONSTRUCTOR_NO_IMPLICIT_ZERO (t))
+	field = next_initializable_field (TYPE_FIELDS (TREE_TYPE (t)));
+      else
+	field = NULL_TREE;
+      FOR_EACH_CONSTRUCTOR_ELT (CONSTRUCTOR_ELTS (t), i, idx, val)
 	{
-	  if (!elt)
+	  if (!val)
 	    /* We're in the middle of initializing this element.  */
 	    return false;
-	  if (!reduced_constant_expression_p (elt))
+	  if (!reduced_constant_expression_p (val))
 	    return false;
+	  if (field)
+	    {
+	      if (idx != field)
+		return false;
+	      field = next_initializable_field (DECL_CHAIN (field));
+	    }
 	}
+      if (field)
+	return false;
+      else if (CONSTRUCTOR_NO_IMPLICIT_ZERO (t))
+	/* All the fields are initialized.  */
+	CONSTRUCTOR_NO_IMPLICIT_ZERO (t) = false;
       return true;
 
     default: