error[E0119]: conflicting implementations of trait `Trait1<Box<_>>` for type `A`
  --> $DIR/issue-43355.rs:13:1
   |
LL | impl<X, T> Trait1<X> for T where T: Trait2<X> {
   | --------------------------------------------- first implementation here
...
LL | impl<X> Trait1<Box<X>> for A {
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `A`
   |
   = note: downstream crates may implement trait `Trait2<std::boxed::Box<_>>` for type `A`

error: aborting due to 1 previous error

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