On Wed, Apr 16, 2014 at 03:33:28PM -0700, Ben Striegel wrote:
> Would this resolve https://github.com/mozilla/rust/issues/3429 ?

Probably not. That particular example involved two blanket impls, one
for all `T:FooBase` and one for all `T:BarBase`. Coherence cannot
currently know whether there exists a type `Foo` that implements both
`FooBase` and `BarBase`. We'd have to add some additional feature,
such as "negative trait bounds" (does not implement this trait) or
some way to declare traits as exclusive (which is basically the same
thing).