error[E0379]: trait fns cannot be declared const
  --> $DIR/issue-54954.rs:7:5
   |
LL |     const fn const_val<T: Sized>() -> usize {
   |     ^^^^^ trait fns cannot be const

error[E0019]: constant contains unimplemented expression type
  --> $DIR/issue-54954.rs:3:24
   |
LL | const ARR_LEN: usize = Tt::const_val::<[i8; 123]>();
   |                        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^

error: aborting due to 2 previous errors

Some errors have detailed explanations: E0019, E0379.
For more information about an error, try `rustc --explain E0019`.