vlib/v/checker/tests/undefined_var_in_comptime_for_test.vv:11:3: error: undefined ident: `fields_len`
    9 | fn test[U](val U) {
   10 |     $for field in U.fields {
   11 |         fields_len++
      |         ~~~~~~~~~~
   12 |         println(field)
   13 |     }
vlib/v/checker/tests/undefined_var_in_comptime_for_test.vv:11:13: error: invalid operation: ++ (non-numeric type `void`)
    9 | fn test[U](val U) {
   10 |     $for field in U.fields {
   11 |         fields_len++
      |                   ~~
   12 |         println(field)
   13 |     }
