Ambiguous user defined conversions and inheritance

[dzmitry-lahoda]

Issue

The compiler cannot infer proper cast in the next 2 cases:

    class A : Exception{}
    class B:A{}
    record C()
    {
        public static implicit operator A(C self) => throw new Exception();
    }
    record D(): C
    {
        public static implicit operator B(D self) => throw new Exception();
    }

    record Main() {
        static Main() {
            var d = new D();
            // Ambiguous user defined conversions 'D.implicit operator B(D)' and 'C.implicit operator A(C)' when converting from 'D' to 'Exception' [Matchmaking]csharp(CS0457)
            throw (A)d;
        }
        // Ambiguous user defined conversions 'D.implicit operator B(D)' and 'C.implicit operator A(C)' when converting from 'D' to 'Exception' [Matchmaking]csharp(CS0457)
        static A a => new D();
        static B b => new D();
    }

Question

Could the compiler compile throw (A)d; and/or static A a => new D(); because I directly asked it what typecast I want?

Why

C# has no multiple inheritances, but it got interfaces with implementations, extensions, and static interfaces.

But sometimes I feel the need to express multiple inheritances so. Sometimes I can use implicit casts.

So also they feel like if I have inherited, for some reason compiler cannot decide what cast to choose even if it has direct suggestions of what to choose.

If cast can be decided, it can make the lack of multiple inheritances even less visible.

Notes

I have seen similar cases where put into csharplang, not roslyn. So I have put it there too.

[CyrusNajmabadi]

because I directly asked it what typecast I want?

You directly asked, but there is no corresponding conversion operator defined for that. Instead, there are two potential different conversions that could lead to that. Specifically:

(A)(B)d or (A)(C)d

So it doesn't know which is better and gives up.

[CyrusNajmabadi]

@dzmitry-lahoda sorry, I'm not sure what your trying to say.

[dzmitry-lahoda]

Specifically how you ensure that any of the N paths that are possible result in the same value and type and runtime.

This is not needed and is impossible. Never asked for that.

[CyrusNajmabadi]

@dzmitry-lahoda you said:

Correct implementation of a pattern is guided to make sure people do not implciit to wrong things.

I'm asking what that guidance would be and what the correct implementation of this pattern would be in this example you're presenting here. Basically, it's not readily apparent how that would work, so i'm basically stating: "please show your work" so that i can better understand what you're proposing.

Thanks!

[CyrusNajmabadi]

@dzmitry-lahoda

As before, can you please show a realistic example of these hierarchies, and how your implicit operators would work here? Having vague A/B/C/D cases isn't helpful as it's too divorced from real work cases. We need something concrete to look at to get an intuition on how this would work and why the proposed rules and guidance would be suitable and appropriate.

This is not needed and is impossible. Never asked for that.

@dzmitry-lahoda You asked for changes to the language. Part of the process around that is showing real examples of how this would work out so we can assess the value here and understand the pros and cons. Asking for examples is not impossible, nor is it unreasonable to ask for this given that you said:

there should be guidance telling to that implicit operators should be coded in predictable way. actually, in case provided they do predictable way. and actually there is already guidance to make implicit predictable.

I'm asking specifically what that guidance would be and how it would be applied in the context of a real world example. Right now your example is too vague, and every attempt at understanding this space better has been met with resistance and a seeming unwillingness to on your part to provide any more information. If you would like to see any change happen here, please provide this information as otherwise this just isn't actionable in its current form.

[dzmitry-lahoda]

I am ok with the answers. I not going to make a proposal as it requires all that you said. Examples, rules, and guidance. In this case, in can be a waste of effort as it is not yet known if others could hit same limit and will try same design and will desire same solution. So better just wait for votes, at lost for me.