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+++ b/src/testdir/test_flatten.vim
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+" Test for flatting list.
+func Test_flatten()
+  call assert_fails('call flatten(1)', 'E686:')
+  call assert_fails('call flatten({})', 'E686:')
+  call assert_fails('call flatten("string")', 'E686:')
+  call assert_fails('call flatten([], [])', 'E745:')
+  call assert_fails('call flatten([], -1)', 'E900: maxdepth')
+
+  call assert_equal([], flatten([]))
+  call assert_equal([], flatten([[]]))
+  call assert_equal([], flatten([[[]]]))
+
+  call assert_equal([1, 2, 3], flatten([1, 2, 3]))
+  call assert_equal([1, 2, 3], flatten([[1], 2, 3]))
+  call assert_equal([1, 2, 3], flatten([1, [2], 3]))
+  call assert_equal([1, 2, 3], flatten([1, 2, [3]]))
+  call assert_equal([1, 2, 3], flatten([[1], [2], 3]))
+  call assert_equal([1, 2, 3], flatten([1, [2], [3]]))
+  call assert_equal([1, 2, 3], flatten([[1], 2, [3]]))
+  call assert_equal([1, 2, 3], flatten([[1], [2], [3]]))
+
+  call assert_equal([1, 2, 3], flatten([[1, 2, 3], []]))
+  call assert_equal([1, 2, 3], flatten([[], [1, 2, 3]]))
+  call assert_equal([1, 2, 3], flatten([[1, 2], [], [3]]))
+  call assert_equal([1, 2, 3], flatten([[], [1, 2, 3], []]))
+  call assert_equal([1, 2, 3, 4], flatten(range(1, 4)))
+
+  " example in the help
+  call assert_equal([1, 2, 3, 4, 5], flatten([1, [2, [3, 4]], 5]))
+  call assert_equal([1, 2, [3, 4], 5], flatten([1, [2, [3, 4]], 5], 1))
+
+  call assert_equal([0, [1], 2, [3], 4], flatten([[0, [1]], 2, [[3], 4]], 1))
+  call assert_equal([1, 2, 3], flatten([[[[1]]], [2], [3]], 3))
+  call assert_equal([[1], [2], [3]], flatten([[[1], [2], [3]]], 1))
+  call assert_equal([[1]], flatten([[1]], 0))
+
+  " Make it flatten if the given maxdepth is larger than actual depth.
+  call assert_equal([1, 2, 3], flatten([[1, 2, 3]], 1))
+  call assert_equal([1, 2, 3], flatten([[1, 2, 3]], 2))
+
+  let l:list = [[1], [2], [3]]
+  call assert_equal([1, 2, 3], flatten(l:list))
+  call assert_equal([1, 2, 3], l:list)
+
+  " Tests for checking reference counter works well.
+  let l:x = {'foo': 'bar'}
+  call assert_equal([1, 2, l:x, 3], flatten([1, [2, l:x], 3]))
+  call test_garbagecollect_now()
+  call assert_equal('bar', l:x.foo)
+
+  let l:list = [[1], [2], [3]]
+  call assert_equal([1, 2, 3], flatten(l:list))
+  call test_garbagecollect_now()
+  call assert_equal([1, 2, 3], l:list)
+
+  " Tests for checking circular reference list can be flatten.
+  let l:x = [1]
+  let l:y = [x]
+  let l:z = flatten(l:y)
+  call assert_equal([1], l:z)
+  call test_garbagecollect_now()
+  let l:x[0] = 2
+  call assert_equal([2], l:x)
+  call assert_equal([1], l:z) " NOTE: primitive types are copied.
+  call assert_equal([1], l:y)
+
+  let l:x = [2]
+  let l:y = [1, [l:x], 3] " [1, [[2]], 3]
+  let l:z = flatten(l:y, 1)
+  call assert_equal([1, [2], 3], l:z)
+  let l:x[0] = 9
+  call assert_equal([1, [9], 3], l:z) " Reference to l:x is kept.
+  call assert_equal([1, [9], 3], l:y)
+
+  let l:x = [1]
+  let l:y = [2]
+  call add(x, y) " l:x = [1, [2]]
+  call add(y, x) " l:y = [2, [1, [...]]]
+  call assert_equal([1, 2, 1, 2], flatten(l:x, 2))
+  call assert_equal([2, l:x], l:y)
+endfunc