error[E0117]: only traits defined in the current crate can be implemented for arbitrary types
  --> $DIR/complex-impl.rs:9:1
   |
LL | impl<R> External for (Q, R) {}
   | ^^^^^^^^^^^^^^^^^^^^^------
   | |                    |
   | |                    this is not defined in the current crate because tuples are always foreign
   | impl doesn't use only types from inside the current crate
   |
   = note: define and implement a trait or new type instead

error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0117`.