error[E0404]: expected trait, found derive macro `Hash`
  --> $DIR/issue-37534.rs:1:16
   |
LL | struct Foo<T: ?Hash> { }
   |                ^^^^ not a trait
help: possible better candidate is found in another module, you can import it into scope
   |
LL | use std::hash::Hash;
   |

warning: default bound relaxed for a type parameter, but this does nothing because the given bound is not a default. Only `?Sized` is supported
  --> $DIR/issue-37534.rs:1:12
   |
LL | struct Foo<T: ?Hash> { }
   |            ^

error[E0392]: parameter `T` is never used
  --> $DIR/issue-37534.rs:1:12
   |
LL | struct Foo<T: ?Hash> { }
   |            ^ unused parameter
   |
   = help: consider removing `T` or using a marker such as `std::marker::PhantomData`

error: aborting due to 2 previous errors

Some errors have detailed explanations: E0392, E0404.
For more information about an error, try `rustc --explain E0392`.