[neonsunset]

The sibling comment looks way closer to port of Go which is rather unidiomatic C#. I understand where it is coming from, but there is a much terser and type-safe way to express it:

    #pragma warning disable CS8509 // The switch expression does not handle all possible values.
    // Justification: The switch expression is exhaustive, and Roslyn emits optimal default guard.
    // In AOT, ILC should be able to statically prove by whole program view that only two subclasses of Tree exist and optimize it away.
    // In practice, it seems null check and recursive nature of the call prevent it for now. It is still compact and fast codegen however.

    var treeWith3Leaves = new Tree.Node(
        new Tree.Leaf(),
        new Tree.Node(
            new Tree.Leaf(),
            new Tree.Leaf()));

    var leaves = NumberOfLeaves(treeWith3Leaves);
    if (leaves != 3) {
        throw new Exception("Huh, something went wrong.");
    }

    static int NumberOfLeaves(Tree tree) {
        return tree switch {
            Tree.Leaf   => 1,
            Tree.Node n => NumberOfLeaves(n.Left) + NumberOfLeaves(n.Right)
        };
    }

    abstract record Tree {
        public record Leaf: Tree;
        public record Node(Tree Left, Tree Right): Tree;
    }

Once we have type unions[0], the syntax could be easily promoted (assuming current proposal state) to e.g.

    union Tree {
        Leaf;
        Node(Tree Left, Tree Right);
    }

Or a bit lower-level variant:

    record Node(Tree Left, Tree Right);

    union struct Tree {
        Leaf;
        Node;
    }

This will allow to remove exhaustiveness suppression and not pay for (house of) leaves.

[0]: https://github.com/dotnet/csharplang/blob/18a527bcc1f0bdaf54...