Derives Wrapper and From fails on generics with defaults

[dr-orlovsky]

error[E0658]: associated type bounds are unstable
    |
144 | pub struct Ty<Ref: TypeRef = SubTy>(TyInner<Ref>);
    |               ^^^^^^^^^^^^
    |
    = note: see issue #52662 <https://github.com/rust-lang/rust/issues/52662> for more information

error[E0229]: associated type bindings are not allowed here
    |
144 | pub struct Ty<Ref: TypeRef = SubTy>(TyInner<Ref>);
    |               ^^^^^^^^^^^^ associated type not allowed here

However

pub struct Ty<Ref: TypeRef>(TyInner<Ref>) where Ref: SubTy;

works