diff --git a/src/libsyntax_ext/deriving/generic/mod.rs b/src/libsyntax_ext/deriving/generic/mod.rs
index 6654afd366465..6dac2f5f9e515 100644
--- a/src/libsyntax_ext/deriving/generic/mod.rs
+++ b/src/libsyntax_ext/deriving/generic/mod.rs
@@ -758,7 +758,7 @@ impl<'a> TraitDef<'a> {
 
 fn find_repr_type_name(diagnostic: &Handler,
                        type_attrs: &[ast::Attribute]) -> &'static str {
-    let mut repr_type_name = "i32";
+    let mut repr_type_name = "isize";
     for a in type_attrs {
         for r in &attr::find_repr_attrs(diagnostic, a) {
             repr_type_name = match *r {
diff --git a/src/test/compile-fail/enum-discrim-autosizing.rs b/src/test/compile-fail/enum-discrim-autosizing.rs
new file mode 100644
index 0000000000000..3a243330ad9b0
--- /dev/null
+++ b/src/test/compile-fail/enum-discrim-autosizing.rs
@@ -0,0 +1,20 @@
+// Copyright 2016 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// With no repr attribute the discriminant will default to isize.
+// On 32-bit architectures this is equivalent to i32 so the variants
+// collide. On other architectures we need compilation to fail anyway,
+// so force the repr.
+#[cfg_attr(not(target_pointer_width = "32"), repr(i32))]
+enum Eu64 {
+    Au64 = 0,
+    Bu64 = 0x8000_0000_0000_0000 //~ERROR already exists
+}
+