error[E0005]: refutable pattern in local binding: `T(_, _)` not covered
  --> $DIR/empty-never-array.rs:10:9
   |
LL | / enum Helper<T, U> {
LL | |     T(T, [!; 0]),
   | |     - not covered
LL | |     #[allow(dead_code)]
LL | |     U(U),
LL | | }
   | |_- `Helper<T, U>` defined here
...
LL |       let Helper::U(u) = Helper::T(t, []);
   |           ^^^^^^^^^^^^ pattern `T(_, _)` not covered
   |
   = note: `let` bindings require an "irrefutable pattern", like a `struct` or an `enum` with only one variant
   = note: for more information, visit https://doc.rust-lang.org/book/ch18-02-refutability.html
   = note: the matched value is of type `Helper<T, U>`
help: you might want to use `if let` to ignore the variant that isn't matched
   |
LL |     if let Helper::U(u) = Helper::T(t, []) { /* */ }
   |

error: aborting due to previous error

For more information about this error, try `rustc --explain E0005`.