error: conflicting implementations of trait `Trait1<std::boxed::Box<_>>` for type `A`: (E0119)
  --> $DIR/issue-43355.rs:23:1
   |
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: #[deny(incoherent_fundamental_impls)] on by default
   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
   = note: for more information, see issue #46205 <https://github.com/rust-lang/rust/issues/46205>
   = note: downstream crates may implement trait `Trait2<std::boxed::Box<_>>` for type `A`

error: aborting due to previous error