error[E0119]: conflicting implementations of trait `Foo`
  --> $DIR/issue-72845.rs:44:1
   |
LL | impl<T: Spec1> Foo for T {
   | ------------------------ first implementation here
...
LL | impl<T: Spec2> Foo for T {
   | ^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation

error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0119`.