error[E0562]: `impl Trait` only allowed in function and inherent method return types, not in type --> $DIR/issue-83929-impl-trait-in-generic-default.rs:1:16 | LL | struct Foo<T = impl Copy>(T); | ^^^^^^^^^ error[E0562]: `impl Trait` only allowed in function and inherent method return types, not in type --> $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>; | ^^^^^^^^^^^^^^^^^^^^^^ error: aborting due to 2 previous errors For more information about this error, try `rustc --explain E0562`.