figure out why we get ambiguity for this futures example

[nikomatsakis]

Given the traits/types found in this gist:

https://gist.github.com/nikomatsakis/58717b0588393ca14f2d6168e7c86188

Why do we get ambiguity here?

> target/debug/chalki --program=$HOME/tmp/futures-model.chalk --goal='forall<T> { if (T: FutureResult) { exists<I, E> { T: Future<Output = Result<I, E>> } } }'
Ambiguous; no inference guidance

Note that if we (hackily) don't force the solver to give us back a result for I and E, we get Unique as expected:

> target/debug/chalki --program=$HOME/tmp/futures-model.chalk --goal='forall<T> { if (T: FutureResult) { T: Future, exists<I, E> { T: Future<Output = Result<I, E>> } } }'
Unique; substitution [], lifetime constraints []

cc @scalexm, who wanted to investigate this

[krk]

Goal no longer gives ambiguous as a result with ef45646:

?- forall<T> { if (T: FutureResult) { exists<I, E> { T: Future<Output = Result<I, E>> } } }
Unique; substitution [?0 := (FutureResult::Item)<!1_0>, ?1 := (FutureResult::Error)<!1_0>], lifetime constraints []