Unions Test Plan

Championed proposal: dotnet/csharplang#9662
Speclet: https://github.com/dotnet/csharplang/blob/main/proposals/unions.md
Feature branch: https://github.com/dotnet/roslyn/tree/features/Unions

• Interactions with "pure null tests" in is-patterns, e.g.:
  • MyUnion myUnion = null; Console.WriteLine(myUnion is object);
  • MyUnion myUnion = new MyUnion(new object()); Console.WriteLine(myUnion is object);
  • MyUnion myUnion = new MyUnion(""); Console.WriteLine(myUnion is object);
  • MyUnion myUnion = null; if (myUnion is object) { myUnion.ToString(); } else { myUnion.ToString(); }
• caching for UnionCaseTypes?
• Unconventional UnionAttribuite definitions, like Use attribute to designate Union types #82408 (comment)
• Exhaustiveness
  • Unions containing union types as case types, where the nested union is exhausted by checking the nested case types. e.g. union Parent(int, Child); union Child(string, bool); switch (new Parent()) { int => 1, string => 2, bool => 3 }
• nullability analysis
  • IUnion.Value without nullable annotation (object!)
• BCL: add IUnion and any other supporting types
• Ping F# as FYI
• Review public APIs
• It would also be good to test the MaybeNull postcondition through the conversion as well.
• WhitespaceNormalizer
• Named parameters, e.g. union MyUnion(string str);
• Parameter modifiers, attributes, and default values
• Redundant Union attribute [Union] union MyUnion(string str);
• Public API: confirm desired shape of declaration syntax

Productivity

Union declarations

• completion (keyword union, type list, union members)
• classfication
• FAR
• NavBar, FindSymbols
• formatting
• disable GenerateConstructor? Start with var x = new MyUnion(""); union MyUnion(int) { }
• UpgradeProject
• offer to upgrade from union type?

Union types

• do we want to display a list of possible types in the union?