test_that("empty input returns a list, but after the index size check", {
  expect_equal(phop_index(list(integer(), integer()), integer(), integer(), integer(), ~.x), list())
  expect_equal(phop_index(list(integer(), 1), integer(), integer(), integer(), ~.x), list())
  expect_equal(phop_index(list(1, integer()), integer(), integer(), integer(), ~.x), list())

  expect_error(phop_index(list(integer(), integer()), 1, integer(), integer(), ~.x), class = "slider_error_index_incompatible_size")
})

test_that("completely empty input returns a list", {
  expect_equal(phop_index(list(), integer(), integer(), integer(), ~.x), list())
})

test_that("empty `.l` and `.i`, but size `n > 0` `.starts` and `.stops` returns size `n` ptype", {
  expect_equal(phop_index(list(), integer(), 1:2, 2:3, ~2), list(2, 2))
})

test_that("can't access non-existant `.x` with empty `.l` and `.i`, but size `n > 0` `.starts` and `.stops`", {
  # Note: Error message seems platform dependent
  expect_error(phop_index(list(), integer(), 1:2, 2:3, ~.x))
})

test_that("empty `.l` and `.i`, but size `n > 0` `.starts` and `.stops`: sizes and types are checked first", {
  expect_snapshot({
    (expect_error(phop_index(list(), integer(), 1:3, 1:2, ~.x), class = "vctrs_error_incompatible_size"))
    (expect_error(phop_index(list(), integer(), 1, "x", ~.x), class = "vctrs_error_incompatible_type"))
  })
})