This was a concern of mine as well, as you can see in the discussion of scope changes for pattern variables [1], the C# checks for definite assignments handle t

This was a concern of mine as well, as you can see in the discussion of scope changes for pattern variables [1], the C# checks for definite assignments handle this. So: if (o is int i) { f(i); } use_int(i); // <-- error: variable `i` is not definitely assigned But then if you have: if (o is int i || (o is string s and int.TryParse(out i)) use_int(i); We know that `i` is definitely assigned at that location. [1]: https://github.com/dotnet/roslyn/issues/12597 https://github.com/dotnet/roslyn/issues/12597