Function with impossible-to-satisfy higher-ranked bound triggers error when a closure is present

[Aaron1011]

The following program compiles successfully:

fn impossible() where for<'a> &'a (): 'static {

}

fn main() {}

The where clause on impossible is impossible to satisfy, since it is not the case that every choice of 'a outlives 'static. However, we never call impossible, so this program compiles (similar to an unsatisfiable trait bound like where String: Copy).

If we add a closure to the function:

fn impossible() where for<'a> &'a (): 'static {
    let _ = || {};
}

fn main() {}

then it stops compiling (note that this occurs both with and without #![feature(nll)]):

error: higher-ranked lifetime error
 --> src/main.rs:2:13
  |
2 |     let _ = || {};
  |             ^^^^^
  |
  = note: could not prove for<'a> &'a (): 'static

error: could not compile `playground` due to previous error

The issue occurs here:

rust/compiler/rustc_borrowck/src/type_check/mod.rs

Line 2688 in 6dc08b9

tcx.predicates_of(def_id).instantiate(tcx, substs)

When we instantiate the predicates of the closure during type-checking of impossible, we also instantiate the predicates for the parent of the closure - that is, the predicates of impossible. This results in us trying to prove that for<'a> &'a (): 'static holds, leading to an error. Normally, we will not try to prove this predicate during type-checking of impossible itself.