[Bug] Double-evaluation of selector in SelectMany

[jamesqo]

I found out my changes from dotnet/corefx#14675 introduced a double-evaluation of a selector in SelectMany.ToArray: https://github.com/dotnet/corefx/blob/master/src/System.Linq/src/System/Linq/SelectMany.cs#L245 This will be fixed shortly in an upcoming PR, but I am opening a tracking issue just in case I forget or that PR gets closed.

[stephentoub]

Thanks, @jamesqo. I thought we'd added tests to help avoid such bugs... I guessed we missed some cases. It'd be great to get a more thorough set for this. cc: @JonHanna

[JonHanna]

You put forward the idea of having tests that both guarded against double evaluation of enumerables and against changes to how often delegates where called. At the time you said that I was already working on the first part of that, and submitted a PR saying I'd look at the other part "soon". Soon has yet to happen.

[stephentoub]

Ah, gotcha, I misremembered. Thanks!

[JonHanna]

Or you remembered my saying "soon" too well. It's definitely something we should have, but ensuring it has coverage would be a bit tricky. Maybe a trait could help there.