error[E0562]: `impl Trait` is not allowed in generic parameter defaults
  --> $DIR/issue-83929-impl-trait-in-generic-default.rs:1:16
   |
LL | struct Foo<T = impl Copy>(T);
   |                ^^^^^^^^^
   |
   = note: `impl Trait` is only allowed in arguments and return types of functions and methods

error[E0562]: `impl Trait` is not allowed in generic parameter defaults
  --> $DIR/issue-83929-impl-trait-in-generic-default.rs:4:20
   |
LL | type Result<T, E = impl std::error::Error> = std::result::Result<T, E>;
   |                    ^^^^^^^^^^^^^^^^^^^^^^
   |
   = note: `impl Trait` is only allowed in arguments and return types of functions and methods

error: aborting due to 2 previous errors

For more information about this error, try `rustc --explain E0562`.