src/test/ui/unboxed-closures/unboxed-closure-no-cyclic-sig.rs

// Test that unboxed closures cannot capture their own type.
//
// Also regression test for issue #21410.

fn g<F>(_: F) where F: FnOnce(Option<F>) {}

fn main() {
    g(|_| {  }); //~ ERROR closure/generator type that references itself
}