NLL type-checking ICEs on closure recursing on impl Trait-returning enclosing function.

[eddyb]

This compiles as expected:

#![feature(nll)]
fn foo() -> impl Copy { || bar(); }
fn bar() -> impl Copy { || foo(); }

This, however, does not, since today (suspecting it's #53542):

#![feature(nll)]
fn foo() -> impl Copy { || foo(); }
fn bar() -> impl Copy { || bar(); }

Each inner closure, because it calls its parent function, causes this ICE: "equate_inputs_and_outputs: impl std::marker::Copy==impl std::marker::Copy failed with NoSolution".
(you only need one of foo or bar, but I left both in for contrast with the first example)

I suspect each impl Copy is instantiated in two different ways, causing inconsistencies.

Originally found through a daily Travis CI build job.