Casts not being applied to switch expression case bodies when type is ambiguous

[nikitalita]

When something is done with the result of the switch expression before it is assigned/returned (e.g. call a member function), and the type is ambiguous (e.g. case bodies have multiple child types of a parent type), we have to apply a cast to at least one of the case bodies to tell the compiler which type it is, or it's a compile error.

Input code

public abstract class Animal
{
    public abstract string Name { get; }

    public Animal ToValidated()
    {
        if (Name is null)
        {
            throw new ArgumentNullException(nameof(Name));
        }
        return this;
    }
}

public class Dog : Animal
{
    public override string Name => "Dog";
}

public class Fish : Animal
{
    public override string Name => "Fish";
}

public class TestAnimals
{
    public Animal GetAnimal(int animalType)
    {
        return (animalType switch
        {
            0 => (Animal)new Dog(),
            1 => new Fish(),
            _ => throw new ArgumentException("Invalid animal type")
        }).ToValidated();
    }
}

Erroneous output

[...]
public class TestAnimals
{
    public Animal GetAnimal(int animalType)
    {
        return (animalType switch
        {
            0 => new Dog(), // no cast, compile error
            1 => new Fish(),
            _ => throw new ArgumentException("Invalid animal type")
        }).ToValidated();
    }
}

For reference, the specific compiler error when attempting to compile the output:

No best type was found for the switch expression.[CS8506](https://msdn.microsoft.com/query/roslyn.query?appId=roslyn&k=k(CS8506))

Details

• Product in use: IlspyCmd
• Version in use: current master

[nikitalita]

I have a fix for this in gdsdecomp, but I am not sure if a transformer on the AST tree is appropriate here; what do you think? https://github.com/GDRETools/gdsdecomp/blob/master/godot-mono-decomp/GodotMonoDecomp/FixSwitchExpressionCasts.cs