Overloaded call inferred to be through Fn when only FnOnce works

[causal-agent]

Repro: https://is.gd/AbO8K4

A wrapper around a closure F implements FnOnce, FnMut and Fn where F does. However, when the wrapper is created with a closure which implements only FnOnce, and then the wrapper is called, the compiler expects the closure to implement FnMut or Fn.

rustc 1.15.0-nightly (daf8c1dfc 2016-12-05)
binary: rustc
commit-hash: daf8c1dfce3b448fc581cc319f64632ec22bd0e1
commit-date: 2016-12-05
host: x86_64-apple-darwin
release: 1.15.0-nightly
LLVM version: 3.9

cc @eddyb (discussed on IRC)

[eddyb]

cc @nikomatsakis @nagisa Is this one of the closure inference cases where the current algorithm is suboptimal? AFAICT the wrong trait is inferred at the callsite and it's not changed later, or even if it's changed to FnOnce there might be some obligation left around.

Error for posterity:

error[E0525]: expected a closure that implements the `Fn` trait, but this closure only implements `FnOnce`
  --> <anon>:31:26
   |
31 |     let f = Wrapper { f: || foo };
   |                          ^^^^^^
   |
note: the requirement to implement `Fn` derives from here
  --> <anon>:32:5
   |
32 |     f();
   |     ^^^

[causal-agent]

Looks like this also applies to closures only implementing FnMut and the compiler wants it to be Fn: https://is.gd/7U8lK6

[nikomatsakis]

triage: P-medium

[nikomatsakis]

I'll try to investigate.

[shepmaster]

Perhaps related to #26186 ?

[Enselic]

Triage: Doesn't seem like it, because this still reproduces even though #26186 is fixed.