Recusion limit (selection ambiguity) reached quickly in monomorphization

[alexcrichton]

Originally reported at rust-lang/futures-rs#393 I've minimized this example to a one-file reproduction.

I'm not really sure why it's hitting the limit so quickly, but is this a bug in the compiler or perhaps a bug in the futures crate?

[alexcrichton]

cc @rust-lang/compiler

[Mark-Simulacrum]

Captured a debug log from rustc_trans: https://gist.github.com/Mark-Simulacrum/55341cf99db2ab2c0739ead0533b7f28

[nikomatsakis]

Nominating for discussion. @alexcrichton how serious of a problem is this? (i.e., is it occurring a lot?)

[alexcrichton]

@nikomatsakis I haven't myself run into this much, but maybe @conradev (reporter of the original issue) has run into it more?

[conradev]

I have only run into it in the situation from the original ticket (rust-lang/futures-rs#393).

That said, @dwrensha also ran into the same error, but in a different situation: rust-lang/futures-rs/issues/367 (I'm not sure how related it is, if at all)

[dwrensha]

I was able to shuffle things around a bit in my code to avoid the problem: dwrensha/zillions@da6e69c. Still, this is a very surprising and unhelpful error message to see, and I can imagine beginners getting quite confused upon seeing it.

[nikomatsakis]

triage: P-medium