[EXTERNAL] Creating variable of Span<> type results in CS8345 error

[ptasz3k]

Steps to duplicate:

var bytes = new byte[10];
Span<byte> span = bytes;

result:

(1,1): error CS8345: Field or auto-implemented property cannot be of type 'Span<byte>' unless it is an instance member of a ref struct.

[jonsequitur]

This happens because what appears to be a local variable in C# scripting is implemented as a field.

[jonsequitur]

Here's the related issue in Roslyn: dotnet/roslyn#40213