A sequence of `union()` should not produce subqueries

[mgirlich]

library(dplyr, warn.conflicts = FALSE)
library(dbplyr, warn.conflicts = FALSE)

lf1 <- lazy_frame(x = 1, y = 1, .name = "lf1")
lf2 <- lazy_frame(y = 1, .name = "lf2")
lf3 <- lazy_frame(z = 1, .name = "lf3")


lf1 %>%
  union(lf2) %>% 
  union(lf3)
#> <SQL>
#> (
#>   SELECT *, NULL AS `z`
#>   FROM (
#>     (
#>       SELECT *
#>       FROM `lf1`
#>     )
#>     UNION
#>     (
#>       SELECT NULL AS `x`, *
#>       FROM `lf2`
#>     )
#>   ) `q01`
#> )
#> UNION
#> (
#>   SELECT NULL AS `x`, NULL AS `y`, *
#>   FROM `lf3`
#> )

^{Created on 2023-04-28 with reprex v2.0.2}