Thanks for assigning this issue to me. I'm starting to work on it and will keep this PR's description up to date as I form a plan and make progress.

Original issue description:

They say there are 4 cases for type pattern matching. But the second and the third seem to me to be identical:

2. expr is an instance of a type that derives from type. In other words, the result of expr can be upcast to an instance of type.

3. expr has a compile-time type that is a base class of type, and expr has a runtime type that is type or is derived from type. The compile-time type of a variable is the variable's type as defined in its declaration. The runtime type of a variable is the type of the instance that is assigned to that variable.

Am I missing something? If there is a very subtle difference, focus should be put on it because this way it's very difficult to spot it.

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Document Details

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• ID: 3126aaa9-8f5f-04f3-2d75-96477b6273cd
• Version Independent ID: 3f4b3eac-11eb-2dd6-4f27-b24537367288
• Content: is - C# Reference
• Content Source: docs/csharp/language-reference/keywords/is.md
• Product: dotnet-csharp
• Technology: csharp-language-reference
• GitHub Login: @BillWagner
• Microsoft Alias: wiwagn

Fixes #22163.

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