error[E0277]: cannot add `<T as SubEncoder>::ActualSize` to `<T as SubEncoder>::ActualSize`
  --> $DIR/issue-54108.rs:19:5
   |
LL |     type Size: Add<Output = Self::Size>;
   |                ------------------------ required by this bound in `Encoder::Size`
...
LL |     type Size = <Self as SubEncoder>::ActualSize;
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ no implementation for `<T as SubEncoder>::ActualSize + <T as SubEncoder>::ActualSize`
   |
   = help: the trait `Add` is not implemented for `<T as SubEncoder>::ActualSize`
help: consider further restricting the associated type
   |
LL |     T: SubEncoder, <T as SubEncoder>::ActualSize: Add
   |                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

error: aborting due to previous error

For more information about this error, try `rustc --explain E0277`.