Type inference regression in rustc 1.18

[khuey]

https://github.com/khuey/rustc-1.18-regression

I'm not sure that this is a minimal testcase, but it's small enough to poke around at. With rustc 1.17 this compiles fine. With 1.18 I get

error[E0284]: type annotations required: cannot resolve `<_ as futures::IntoFuture>::Future == _`
  --> src/lib.rs:15:18
   |
15 |           Box::new(join_all(vec2.iter()
   |  __________________^
16 | |                           .map(move |_| {
17 | |                               iter::<_, _, ()>(vec1.iter()
18 | |                                                .filter_map(move |_| {
...  |
24 | |                                   })
25 | |                           })))

[frewsxcv]

For posterity, this is also broken on the latest nightly as well

rustc 1.19.0-nightly (f062832b2 2017-06-07)
binary: rustc
commit-hash: f062832b208e94f2f0f26ed7fb5c48c172069fbe
commit-date: 2017-06-07
host: x86_64-apple-darwin
release: 1.19.0-nightly
LLVM version: 4.0

[nikomatsakis]

triage: P-high -- regression

[Mark-Simulacrum]

Bisected to 141e8a6, #41716 -- cc @nikomatsakis.

[nikomatsakis]

Well, I can certainly reproduce it =)

[nikomatsakis]

This may be a dup of #42552

[nikomatsakis]

My fix for #42552 also fixes this.