error[E0119]: conflicting implementations of trait `A`
  --> $DIR/specializing-constness.rs:23:1
   |
LL | impl<T: ~const Spec> const A for T {
   | ---------------------------------- first implementation here
...
LL | impl<T: Spec + Sup> A for T {
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation

error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0119`.