Add Path.Join string overloads

[MarcoRossignoli]

coreclr part of https://github.com/dotnet/corefx/issues/30049
corefx PR are ready to go dotnet/corefx@master...MarcoRossignoli:joinoverloads

/cc @JeremyKuhne

[JeremyKuhne]

Thanks!

[stephentoub]

These are going to end up treating null strings the same as empty strings. Do we instead want to match the behavior of Path.Combine, which throws ArgumentNullException for null strings?

[JeremyKuhne]

The key thing here is that we've shipped this way already. As is illustrated here, ((string)null).AsSpan() gives you default, it doesn't throw. Same goes for the implicit conversion- presumably adding the throw would be a breaking change for those relying on that.

Outside of that I'd rather they don't throw as they'll never throw for the span versions. I'm more inclined to be consistent within the overloads. I don't plan to ever add a span version to Combine as I want to discourage usage- if I thought that likely I might sway a little, but still not enough to want to throw I think.

@pjanotti do you have an opinion?

[stephentoub]

The key thing here is that we've shipped this way already. As is illustrated here, ((string)null).AsSpan() gives you default, it doesn't throw.

Ugh, you're right. Ok, we should keep it the way it is; otherwise adding these overloads would change behavior against 2.1 when code is recompiled.

[jkotas]

I can do Path.Join(String.Empty, (string)null) today. It compiles and runs fine thanks to implicit string->span conversion. This code will pick up the new string overload once it is added. It is preferable for the new string overload to have same behavior as the Span version to avoid breaking change on recompile.

[stephentoub]

Right, that's what I was agreeing with.