¿Por qué el ejemplo de código 1 compila pero el ejemplo 2 da un error de compilación?

Ejemplo 1:

use std::ops::Index;

struct Bounded {
    idx: usize,
}

impl Index<Bounded> for [i32; 4] {
    type Output = i32;

    fn index(&self, b: Bounded) -> &i32 {
        unsafe { self.get_unchecked(b.idx) }
    }
}

Ejemplo 2

use std::ops::Index;

struct Bounded {
    idx: usize,
}

impl<T> Index<Bounded> for [T; 4] {
    type Output = T;

    fn index(&self, b: Bounded) -> &T {
        unsafe { self.get_unchecked(b.idx) }
    }
}

error[E0210]: type parameter `T` must be used as the type parameter for some local type (e.g. `MyStruct<T>`)
 --> src/main.rs:7:1
  |
7 | impl<T> Index<Bounded> for [T; 4] {
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ type parameter `T` must be used as the type parameter for some local type
  |
  = note: only traits defined in the current crate can be implemented for a type parameter