Multiple different parameterisations of a trait in a bounds list conflict.

[dylanede]

With this preamble:

struct A;
struct B;

trait F<T> {}

this code does not compile:

fn f<T: F<A> + F<B>>() {}

but this does:

fn f<T>() where T : F<A> + F<B> {}

As an example of a case where this can't be worked around, consider associated types. This example does not seem to have any possible workaround:

trait G {
    type X : F<A> + F<B>;
}

This attempt at a workaround causes an ICE:

trait G where Self::X : F<A> + F<B> {
    type X;
}

and this example apparently isn't valid syntax:

trait G {
    type X where X : F<A> + F<B>;
}

[steveklabnik]

I don't know when this was fixed, but everything now compiles, no ICE, all forms. Except for that last one, which is indeed not valid.