// (21,30): error CS8164: Cannot return by reference a result of 'Program.Test2(ref int)' because the argument passed to parameter 'arg' cannot be returned by reference
// (21,24): error CS8164: Cannot return by reference a result of 'Program.Test1(Program.S1)' because the argument passed to parameter 'arg' cannot be returned by reference
// (21,30): error CS8164: Cannot return by reference a result of 'Program.Test2(in int)' because the argument passed to parameter 'arg' cannot be returned by reference
// (21,24): error CS8164: Cannot return by reference a result of 'Program.Test1(Program.S1)' because the argument passed to parameter 'arg' cannot be returned by reference
//PROTOTYPE(span): make this bind-time diagnostic?
//PROTOTYPE(span): need a better error message for invalid span captures (should not say ref)
comp.VerifyDiagnostics(
// (13,26): error CS0701: 'Span<int>' is not a valid constraint. A type used as a constraint must be an interface, a non-sealed class or a type parameter.