error: defaults for generic parameters are not allowed here
  --> $DIR/issue-105257.rs:5:12
   |
LL |     fn fnc<const N: usize = "">(&self) {}
   |            ^^^^^^^^^^^^^^^^^^^

error: defaults for generic parameters are not allowed here
  --> $DIR/issue-105257.rs:7:12
   |
LL |     fn foo<const N: usize = { std::mem::size_of::<T>() }>(&self) {}
   |            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

error[E0308]: mismatched types
  --> $DIR/issue-105257.rs:5:29
   |
LL |     fn fnc<const N: usize = "">(&self) {}
   |                             ^^ expected `usize`, found `&str`

error: aborting due to 3 previous errors

For more information about this error, try `rustc --explain E0308`.