Merely implementing the generic IComparable interface with CustomComparisonAttribute does not satisfy the "comparison" constraint.

[teo-tsirpanis]

I think the title is descriptive enough.

Code sample:

open System

[<StructuralEquality; CustomComparison>]
type MyType = MyType of int
with
  member x.Value = match x with MyType x -> x
  interface IComparable<MyType> with
    member x.CompareTo(x') =
      compare x.Value x'.Value

let mySet = Set.singleton <| MyType 521

At MyType 521, it presents the following compiler error:

error FS0001: The type 'MyType' does not support the 'comparison' constraint. For example, it does not support the 'System.IComparable' interface

MyType however implements the generic IComparable. Why isn't that enough? Shouldn't the compiler provide an automatic implementation for the non-generic interface in cases like that?

On the other hand, the generic IEquatable interface is sufficient for the "equality" constraint.

[cartermp]

This is currently by design - the comparison constraint is satisfied by the System.IComparable interface, not the System.IComparable<'T> interface. I recommend filing a new language suggestion here to propose that the generic interface do this: https://github.com/fsharp/fslang-suggestions/issues