Add regression tests for possible bug in OrderedEnumerable.Last{OrDefault}

[jamesqo]

I am not too familiar with OrderedEnumerable, but while making changes during dotnet/corefx#14174 I noticed there was a discrepancy between Last and LastOrDefault.

Code in Last: https://github.com/dotnet/corefx/blob/master/src/System.Linq/src/System/Linq/OrderedEnumerable.cs#L416. The result of comparer.Compare is compared with >= to 0.

Code in LastOrDefault: https://github.com/dotnet/corefx/blob/master/src/System.Linq/src/System/Linq/OrderedEnumerable.cs#L447 The result of comparer.Compare is compared with > to 0.

I assume this is trying to somehow find the maximum value of the enumerable? If that is the case, is the latter version correct?

cc @JonHanna @VSadov

[jamesqo]

Note: I am making changes to the method in dotnet/corefx#14174 so please wait until that goes in first. Thanks.

[JonHanna]

The second one is incorrect because in the case of Enumerable (unlike for other linq providers) OrderBy is specified as being a stable sort. The combination OrderBy(keySelector).Last(pred) should therefore not only find the value which produces the largest key and matches the predicate, but also in the case of a tie find the last such value. >= does that because it replaces the current match in the case of a tie, > does not.

Note: I am making changes to the method in dotnet/corefx#14174 so please wait until that goes in first. Thanks.

I see that you are changing the first, but not the second. Shall I go ahead and push my fix for the second?

[jamesqo]

@JonHanna Sure, go ahead. I had actually deleted both methods entirely in my editor and then copied-and-pasted, so I assumed this would cause merge conflicts, but it's fortunate that that area is unaffected.