error[E0119]: conflicting implementations of trait `std::ops::Drop` for type `std::boxed::Box<_>`:
  --> $DIR/issue-41974.rs:7:1
   |
LL | impl<T> Drop for T where T: A {
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = note: conflicting implementation in crate `alloc`:
           - impl<T> std::ops::Drop for std::boxed::Box<T>
             where T: ?Sized;
   = note: downstream crates may implement trait `A` for type `std::boxed::Box<_>`

error[E0120]: the Drop trait may only be implemented on structures
  --> $DIR/issue-41974.rs:7:18
   |
LL | impl<T> Drop for T where T: A {
   |                  ^ implementing Drop requires a struct

error[E0210]: type parameter `T` must be used as the type parameter for some local type (e.g., `MyStruct<T>`)
  --> $DIR/issue-41974.rs:7:6
   |
LL | impl<T> Drop for T where T: A {
   |      ^ type parameter `T` must be used as the type parameter for some local type
   |
   = note: implementing a foreign trait is only possible if at least one of the types for which is it implemented is local
   = note: only traits defined in the current crate can be implemented for a type parameter

error: aborting due to 3 previous errors

Some errors have detailed explanations: E0119, E0120, E0210.
For more information about an error, try `rustc --explain E0119`.