Use when a project targets net11.0/C# 15 (or is deciding whether to) and you need the full depth on union types, closed hierarchies, collection expression arguments, extension indexers, labeled break/continue, or the memory-safety preview — implicit conversions, exhaustiveness, what the compiler actually generates, the boxing cost of unions with value-type cases, and how to choose between union/closed/enum/hand-written discriminated union. `csharp-language-versions`, `csharp-pattern-matching`, `csharp-modern-types`, `csharp-extension-members` and `csharp-collections-modern` cover these features briefly; this is the deep-dive.
