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A type parameter which references `Self` in its default value was not specified.
Erroneous code example:
```compile_fail,E0393
trait A<T=Self> {}
fn together_we_will_rule_the_galaxy(son: &A) {}
// error: the type parameter `T` must be explicitly specified in an
// object type because its default value `Self` references the
// type `Self`
```
A trait object is defined over a single, fully-defined trait. With a regular
default parameter, this parameter can just be substituted in. However, if the
default parameter is `Self`, the trait changes for each concrete type; i.e.
`i32` will be expected to implement `A<i32>`, `bool` will be expected to
implement `A<bool>`, etc... These types will not share an implementation of a
fully-defined trait; instead they share implementations of a trait with
different parameters substituted in for each implementation. This is
irreconcilable with what we need to make a trait object work, and is thus
disallowed. Making the trait concrete by explicitly specifying the value of the
defaulted parameter will fix this issue. Fixed example:
```
trait A<T=Self> {}
fn together_we_will_rule_the_galaxy(son: &A<i32>) {} // Ok!
```
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