Deedle FillMissing() function bug?

[DutchL]

I work with Deedle v.1.2.5 on C# and do get strange behavior.

When I replace missing values with FillMissing(double.NaN) function, it works as expected, but FillMissing((x) => double.NaN) code will not replace missing values.

What am I doing wrong?

//unitest
[Test]
public void Missing()
{
    var sb = new SeriesBuilder<int, double>() { { 1, double.NaN }, { 2, 2 } };
    var serie = sb.Series;
    //replaces missing with NaN
    var res = serie.FillMissing(double.NaN);

    //ok
    Assert.AreEqual(double.NaN, res.FirstValue());

    var sb2 = new SeriesBuilder<int, double>() { { 1, double.NaN }, { 2, 2 } };
    var serie2 = sb2.Series;
    //NOT replaces missing with NaN
    var res2 = serie2.FillMissing((x) => double.NaN);

    //crashes
    Assert.AreEqual(double.NaN, res2.FirstValue());
}

[zyzhu]

This is actually a bug in FillMissingWith. It assumes the replacement constant value is not nan. By design, Deedle should always handle nan value as missing value. Otherwise functions such as Stats.sum won't work as expected.

So basically the second case above is the correct behavior.