[swift-evolution] 'T != Type' in where clause

Joe Groff jgroff at apple.com
Tue Feb 28 11:33:13 CST 2017

> On Feb 28, 2017, at 9:23 AM, Matthew Johnson <matthew at anandabits.com> wrote:
>> On Feb 28, 2017, at 11:04 AM, Joe Groff via swift-evolution <swift-evolution at swift.org> wrote:
>>> On Feb 27, 2017, at 4:34 PM, Rex Fenley via swift-evolution <swift-evolution at swift.org> wrote:
>>> I often find myself running into situations where I'll receive "Ambiguous use of..." for overloaded functions or operators. In every case these situations would be easily solved if I could specify "Generic != CertainType" in the where clause of one of the overloads so I can disambiguate the cases. Could this be added to language?
>> Do you have a concrete example where you need this? It'd be good to know whether the types are ambiguous due to type checker bugs, or whether there's a principle by which they could be naturally ordered. Instead of overloading, can you do the type test via `if !(x is CertainType)` within a single implementation?
> The best use case I can think of is if we had enum cases where the associated value is a subtype of the enum:
> enum Result<T, E> where E: Error, T != E {
>    case some(T) -> T
>    case error(E) -> E
> }

I don't think that's a good design for that type. I can see the desire for a subtype relationship between T and Result<T, E>, but no good reason for the error to also be a subtype. That != constraint would have to be propagated through anything using `Result<T, E>` as well.


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