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: &dyn A) {}
// error: the type parameter `T` must be explicitly specified
```

A trait object is defined over a single, fully-defined trait. With a regular
default parameter, this parameter can just be instantiated 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 instantiated 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: &dyn A<i32>) {} // Ok!
```
