Build fails when using type parameter for function in Rust

weird_rust_error.rs

// this compiles
fn validator<F, T>(f: F) -> Box<Fn(String) -> Result<(), String> + 'static>
    where F: Fn(String) -> Result<T,  String> + 'static {
    Box::new(move |s| f(s).map(|_| ()))
}

// this fails with:
// error[E0308]: mismatched types
//   --> src\main.rs:17:14
//    |
// 17 |     Box::new(move |s| f(s).map(|_| ()))
//    |              ^^^^^^^^^^^^^^^^^^^^^^^^^ expected type parameter, found closure
//    |
//    = note: expected type `G`
//               found type `[closure@src\main.rs:17:14: 17:39 f:_]`
// 
// error: aborting due to previous error
// 
// For more information about this error, try `rustc --explain E0308`.
fn validator_bad<F, G, T>(f: F) -> Box<G>
    where F: Fn(String) -> Result<T,  String> + 'static,
          G: Fn(String) -> Result<(), String> + 'static {
    Box::new(move |s| f(s).map(|_| ()))
}