Suppose I have a struct S<T> and I want implement it to trait Default with different constraints of generic type T:
struct S<T> { ... }
impl<T> Default for S<T>
where
T: Ord
{
fn default() -> Self {
Self::new()
}
}
impl<T> Default for S<T>
where
T: Ord + Default
{
fn default() -> Self {
Self::build(T::default())
}
}
However, the compiler complains:
error[E0119]: conflicting implementations of trait std::default::Default for type S<_>
Is there any way to compile such code?
There is no way to do this in stable Rust.
You can use the (unstable and unsound!) specialization feature on nightly Rust:
#![feature(specialization)]
struct S<T> {
x: Option<T>
}
impl<T> S<T> {
fn new() -> Self { Self { x: None } }
fn build(x: T) -> Self { Self { x: Some(x) } }
}
impl<T> Default for S<T>
where
T: Ord
{
default fn default() -> Self {
Self::new()
}
}
impl<T> Default for S<T>
where
T: Ord + Default
{
fn default() -> Self {
Self::build(T::default())
}
}