To be clear, it actually does work with old versions of the C# compiler that predate Span<T>. Newer versions are just able to make use of better compile-time type checks with it. I have no idea why F# chokes on it.
@Dennis Have you considered just removing the mcs version of C#? There's absolutely no reason for anyone to use it. Even if someone did want the old compiler for some reason, they could just set the appropriate langversion flag.
@Dennis Sometimes, yeah. It's nothing too major, but occasionally it takes me a while before I realize that something not working is because mcs is broken and not because I did something wrong.
That said, it's not causing any real issues--it's just pointless.
It's also marked for removal from a future version of mono.
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