warning: `union` should have at least 2 input(s), actually has 0.
 --> tests/compile-fail/surface_degenerate_merge.rs:5:9
  |
5 |         union();
  |         ^^^^^^^

error: `union` must have exactly 1 output(s), actually has 0.
 --> tests/compile-fail/surface_degenerate_merge.rs:5:9
  |
5 |         union();
  |         ^^^^^^^