error[E0091]: type parameter `T` is never used
  --> $DIR/issue-17994.rs:2:10
   |
LL | type Huh<T> where T: Tr = isize;
   |          ^ unused type parameter
   |
   = help: consider removing `T` or referring to it in the body of the type alias

error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0091`.