Managed type system casting logic doesn't handle all array variance

[MichalStrehovsky]

Compile following with optimizations on:

class Program
{
    static void Main()
    {
        object source = new Enum0[] { Enum0.First, Enum0.Second, Enum0.Third };
        Console.WriteLine(source is IEnumerable<Enum1>);
    }

    enum Enum0 { First, Second, Third }
    enum Enum1 { }
}

Expected: prints true
Observed: prints false

This is because the CanCastTo logic within the managed type system doesn't think these are castable. RyuJIT hardcodes this to false in codegen.

[ghost]

Tagging subscribers to this area: @agocke, @MichalStrehovsky, @jkotas
See info in area-owners.md if you want to be subscribed.

Issue Details

---

Compile following with optimizations on:

class Program
{
    static void Main()
    {
        object source = new Enum0[] { Enum0.First, Enum0.Second, Enum0.Third };
        Console.WriteLine(source is IEnumerable<Enum1>);
    }

    enum Enum0 { First, Second, Third }
    enum Enum1 { }
}

Expected: prints true
Observed: prints false

This is because the CanCastTo logic within the managed type system doesn't think these are castable. RyuJIT hardcodes this to false in codegen.

Author:	MichalStrehovsky
Assignees:	-
Labels:	area-NativeAOT-coreclr
Milestone:	-

[Fabi]

Just a thing to clarify: The is keyword actually returns true as soon as an implicit conversion is possible? I thought it only returns true on exact type matches :D

[MichalStrehovsky]

It returns true if a cast can succeed.

You can do ((IEnumerable<Enum1>)source).GetType() and you'll get Enum0[]. There's no conversion involved, just a type check.

[Fabi]

It returns true if a cast can succeed.

You can do ((IEnumerable<Enum1>)source).GetType() and you'll get Enum0[]. There's no conversion involved, just a type check.

That's good to know, thank you. I always expected it to only return true on exact type matches for types you defined.