vlib/v/checker/tests/void_fn_as_value.vv:5:7: error: unknown function: x
    3 | fn main() {
    4 |     mut a := 'aa'
    5 |     a += x('a', 'b')
      |          ~~~~~~~~~~~
    6 |     mut b := 'abcdef'
    7 |     _ = b
vlib/v/checker/tests/void_fn_as_value.vv:5:4: error: assignment mismatch: 1 variable but `x()` returns 0 values
    3 | fn main() {
    4 |     mut a := 'aa'
    5 |     a += x('a', 'b')
      |       ~~
    6 |     mut b := 'abcdef'
    7 |     _ = b