error[E0015]: cannot call non-const fn `<T as Foo>::a` in constant functions
  --> $DIR/super-traits-fail-2.rs:16:7
   |
LL |     x.a();
   |       ^^^
   |
   = note: calls in constant functions are limited to constant functions, tuple structs and tuple variants

error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0015`.