src/test/ui/unboxed-closures/issue-30906.rs

#![feature(fn_traits, unboxed_closures)]

fn test<F: for<'x> FnOnce<(&'x str,)>>(_: F) {}

struct Compose<F,G>(F,G);
impl<T,F,G> FnOnce<(T,)> for Compose<F,G>
where F: FnOnce<(T,)>, G: FnOnce<(F::Output,)> {
    type Output = G::Output;
    extern "rust-call" fn call_once(self, (x,): (T,)) -> G::Output {
        (self.1)((self.0)(x))
    }
}

fn bad<T>(f: fn(&'static str) -> T) {
    test(Compose(f, |_| {})); //~ ERROR: mismatched types
}

fn main() {}