A non-default implementation was already made on this type so it cannot be specialized further.
Erroneous code example:
#![feature(specialization)]
trait SpaceLlama {
fn fly(&self);
}
// applies to all T
impl<T> SpaceLlama for T {
default fn fly(&self) {}
}
// non-default impl
// applies to all `Clone` T and overrides the previous impl
impl<T: Clone> SpaceLlama for T {
fn fly(&self) {}
}
// since `i32` is clone, this conflicts with the previous implementation
impl SpaceLlama for i32 {
default fn fly(&self) {}
// error: item `fly` is provided by an `impl` that specializes
// another, but the item in the parent `impl` is not marked
// `default` and so it cannot be specialized.
}Specialization only allows you to override default functions in
implementations.
To fix this error, you need to mark all the parent implementations as default. Example:
#![feature(specialization)]
trait SpaceLlama {
fn fly(&self);
}
// applies to all T
impl<T> SpaceLlama for T {
default fn fly(&self) {} // This is a parent implementation.
}
// applies to all `Clone` T; overrides the previous impl
impl<T: Clone> SpaceLlama for T {
default fn fly(&self) {} // This is a parent implementation but was
// previously not a default one, causing the error
}
// applies to i32, overrides the previous two impls
impl SpaceLlama for i32 {
fn fly(&self) {} // And now that's ok!
}