error[E0308]: mismatched types
  --> tests/compile/derive-invalid-strategy.rs:8:24
   |
8  |       #[merge(strategy = my_custom_merge_strategy)]
   |                          ^-----------------------
   |                          |
   |  ________________________arguments to this function are incorrect
   | |
9  | |     field1: u8,
   | |__________^ expected `u8`, found `&mut u8`
   |
note: function defined here
  --> tests/compile/derive-invalid-strategy.rs:12:4
   |
12 | fn my_custom_merge_strategy(left: u8, right: u8) -> u8 {
   |    ^^^^^^^^^^^^^^^^^^^^^^^^ --------