error[E0403]: the name `M` is already used for a generic parameter in this item's generic parameters
  --> $DIR/self-referential.rs:1:30
   |
LL | trait Foo<const M: u8, const M: u8 = M> {}
   |                 -            ^ already used
   |                 |
   |                 first use of `M`

error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0403`.