[C# 15-Fundamentals]: New Feature - Closed hierarchies

[BillWagner]

.NET 11 Preview

.NET 11 Preview 5

Link to speclet

• Speclet

Link to Pull request

• PR

New behavior

To quickly follow the .NET 11 release add fundamentals coverage for closed hierarchies.

In addition, compare with unions and the work in #52320

Language reference tasks

Should be covered before this issue.

Everyday C# tasks

• Update pattern matching articles on this.
• Update class and type system discussions.
• Update records and types discussions.

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Associated WorkItem - 582356