Problem with openarrays and slices

[zeitbit]

In this example the output is incorrect, it is @[5, 5, 0, 1, 2, 3, 4, 0, 0, 0] but it should be @[0, 1, 2, 3, 4].

proc test(a: seq[int], b: var openarray[int]) =
    b = a[0 .. 4]

var
    a = @[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
    b = newSeq[int](10)

test(a, b)
echo b

Nim Compiler Version 0.13.1 (2016-05-13) [MacOSX: amd64]

[Parashurama]

Confirmed on Linux latest devel.
Works OK if replacing openarray by seq

However, I not so sure you are supposed to use openarray like that. here is the generated C code.

// code for test(a: seq[int], b: var openarray[int])
-N_NIMCALL(void, test_94003_1385730820)(TY94083* a0, NI* b0, NI b0Len0)

// code for test(a: seq[int], b: var seq[int])
+N_NIMCALL(void, test_94003_1385730820)(TY94083* a0, TY94083** b0)
 }

As you can see an openarray is literally a pointer to the data, and a size. It doen't care what the original object was, and so I pretty sure you are not supposed to assign a seq or a string to it directly.

I think this should be a compile time error.

You might, however, assign data to to its slice with something like this.

proc `[]=`[T](s: var openarray[T]; x: Slice[int]; b: openarray[T]) =
    assert(x.a <= s.high and x.b <= s.high)
    var idx = 0
    for i in x.a..x.b:
        s[i] = b[idx]
        inc(idx)

proc test(a: seq[int], b: var openarray[int]) =
    # assign to openarray slice. original data must be at least slice size.
    b[0 .. 4] = a[0 .. 4]

[zeitbit]

Yes, I know it works with seqs, but if it's not allowed to assign data to openarrays with slices then the compiler should generate an error.