F#中SN类型标注CustomEquality等属性报FS0377错误的原因咨询

报错的类类型代码 #

[<CustomComparison; CustomEquality>]
type SN () =        
    interface IComparable<SN> with
        member this.CompareTo other = failwith ""            
    interface IComparable with
        member this.CompareTo obj = failwith ""
    interface IEquatable<SN> with
        member this.Equals other = failwith ""
    override this.Equals obj = failwith ""
    override this.GetHashCode () = failwith ""

编译时抛出错误：Error FS0377 This type uses an invalid mix of the attributes 'NoEquality', 'ReferenceEquality', 'StructuralEquality', 'NoComparison' and 'StructuralComparison'

正常运行的联合类型代码 #

[<CustomEquality;CustomComparison>]
type DU =
    | L    
    override x.Equals(yobj) = failwith ""
    override x.GetHashCode() = failwith ""
    interface System.IComparable with
        member x.CompareTo yobj = failwith ""

问题原因 #

这是F#编译器的历史bug：当类同时实现IComparable<T>泛型接口与CustomComparison/CustomEquality属性时，编译器会错误地自动推断启用了结构比较/相等属性，这与显式标注的自定义属性产生冲突，进而抛出错误。

而联合类型（DU）不存在这个错误推断逻辑，因此可以正常编译。该bug后续已被修复。

旧版本编译器临时解决办法 #

可以显式添加NoStructuralEquality和NoStructuralComparison属性，抵消编译器的错误推断：

[<CustomComparison; CustomEquality; NoStructuralEquality; NoStructuralComparison>]
type SN () =        
    interface IComparable<SN> with
        member this.CompareTo other = failwith ""            
    interface IComparable with
        member this.CompareTo obj = failwith ""
    interface IEquatable<SN> with
        member this.Equals other = failwith ""
    override this.Equals obj = failwith ""
    override this.GetHashCode () = failwith ""

内容的提问来源于stack exchange，提问作者citykid