error[E0277]: `Foo<T, U>` cannot be sent between threads safely
  --> $DIR/issue-83857-ub.rs:22:38
   |
LL | fn generic<T, U>(v: Foo<T, U>, f: fn(<Foo<T, U> as WithAssoc>::Output) -> i32) {
   |                                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `Foo<T, U>` cannot be sent between threads safely
   |
   = help: the trait `Send` is not implemented for `Foo<T, U>`
note: required for `Foo<T, U>` to implement `WithAssoc`
  --> $DIR/issue-83857-ub.rs:15:15
   |
LL | impl<T: Send> WithAssoc for T {
   |         ----  ^^^^^^^^^     ^
   |         |
   |         unsatisfied trait bound introduced here
help: consider introducing a `where` clause, but there might be an alternative better way to express this requirement
   |
LL | fn generic<T, U>(v: Foo<T, U>, f: fn(<Foo<T, U> as WithAssoc>::Output) -> i32) where Foo<T, U>: Send {
   |                                                                                +++++++++++++++++++++

error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0277`.