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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!
+```