[C++][Dataset] in expressions don't work with >1 partition levels

library(arrow)
#> 
#> Attaching package: 'arrow'
#> The following object is masked from 'package:utils':
#> 
#>     timestamp
library(dplyr)
#> 
#> Attaching package: 'dplyr'
#> The following objects are masked from 'package:stats':
#> 
#>     filter, lag
#> The following objects are masked from 'package:base':
#> 
#>     intersect, setdiff, setequal, union

## Write files
pqdir <- file.path(tempdir(), paste(sample(letters, 6), collapse = ""))

for (foo in 0:1) {
  for (faa in 0:1) {
    fdir <- file.path(pqdir, letters[foo + 1], letters[faa + 1])
    dir.create(fdir, recursive = TRUE)
    rng <- (foo * 5 + faa + 1):(foo * 5 + faa + 5)
    write_parquet(data.frame(col = letters[rng]),
                         file.path(fdir, "file.parquet"))
  }
}

## What doesn't work: using %in% with both partitions defined
ds <- open_dataset(pqdir,
                   partitioning = schema(foo = string(), faa = string()))

collect(filter(ds, foo %in% "a"))
#> # A tibble: 0 x 3
#> # ... with 3 variables: col <chr>, foo <chr>, faa <chr>

## == does work
collect(filter(ds, foo == "a"))
#> # A tibble: 10 x 3
#>    col   foo   faa  
#>    <chr> <chr> <chr>
#>  1 a     a     a    
#>  2 b     a     a    
#>  3 c     a     a    
#>  4 d     a     a    
#>  5 e     a     a    
#>  6 b     a     b    
#>  7 c     a     b    
#>  8 d     a     b    
#>  9 e     a     b    
#> 10 f     a     b

## Declaring only one partition does work
ds <- open_dataset(pqdir, partitioning = schema(foo = string()))
collect(filter(ds, foo %in% "a"))
#> # A tibble: 10 x 2
#>    col   foo  
#>    <chr> <chr>
#>  1 a     a    
#>  2 b     a    
#>  3 c     a    
#>  4 d     a    
#>  5 e     a    
#>  6 b     a    
#>  7 c     a    
#>  8 d     a    
#>  9 e     a    
#> 10 f     a

## The lower-level API has the same problem
ds <- open_dataset(pqdir,
                   partitioning = schema(foo = string(), faa = string()))

flt <- Expression$in_(Expression$field_ref("foo"), Array$create("a"))

sc <- Scanner$create(ds, filter = flt)
sc$ToTable()
#> Table
#> 0 rows x 3 columns
#> $col <string>
#> $foo <string>
#> $faa <string>