Enhance switch expression support for closed classes set (sealed classes/private protected constructor only)

[rmannibucau]

Is your feature request related to a problem? Please describe.

Goal is to avoid the default case when it can't happen

Describe the solution you'd like

Assuming you have a private protected constructor as only way to create an abstract class instance, it means all classes will be defined in the assembly so the compiler know all classes, then switch expression testing the type only shouldn't require default when all types are listed

Additional context

Ex:

public abstract class Base
{
    private protected Base() { }
    public abstract string Whatever();
}

public class Foo : Base { public override string Whatever() => "foo"; }

public class Bar : Base { public override string Whatever() => "bar"; }

public static class Demo
{
    public static void Main(string[] args)
    {
        Base instance = new Foo();
        var test = instance switch
        {
            Foo f => f,
            Bar b => b,
            { } other => other, // this is useless but required today
        };
    }
}

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[KalleOlaviNiemitalo]

Can't the program use the reflection emit API to create another derived class?

[rmannibucau]

@KalleOlaviNiemitalo assuming so, the fact is a standard switch doesn't need that and the program would need to modify the switch anyway to handle the class so compiler really assume build time code and runtime code must ensure it modifies the code properly to keep it working, default case in switch expression is not in this case most of the time for previous case since you designed a closed set of classes and add one so most of the code must be rewritten at runtime with emit api anyway.

[KalleOlaviNiemitalo]

Related: dotnet/csharplang#8942, but that talks about private constructors used by nested classes

[KalleOlaviNiemitalo]

Also dotnet/csharplang#8927, which proposes marking closed class hierarchies with a dedicated keyword in the source code, rather than than inferring that from the accessibility of constructors.