Regression report stable-2015-08-06 vs. nightly-2015-09-04

[jschievink]

This reproduces the influent regression:

trait Tr {}

struct S<'a>(&'a str);

fn f(a: S, b: Box<Tr>) -> S { a }

Works on stable, not on nightly. Using another S instead of the Box<Tr> always fails, as expected.