Nullable with generic types behaves so weird

[Cyl18]

For example, I wrote a method that can get a list's element with index, and returns default when the index is above the list's length.

TValue can be null, so I added a ? to TValue

But this can't be done. It suggests me to add a class or a struct, so I write this method separately:

And this can't be done too...

So I'm wondering if there is a way to achieve my thought?
(um I'm a student and my English isn't quite good, so if there's anything that I didn't express clearly, please point it out!)

[YairHalberstadt]

In C# 9 the first thing you wrote works.

[Cyl18]

I'd already changed to .NET 5 and C# 9, and it still showed that error, but it turns out that it's actually resharper's problem, after I turned off resharper, it works!

thank you!