Duck Typing & Type Augmenttaion

Question

Hello,

I have just noticed a strange behavior when using duck typing and type augmentation. Here is the code:

namespace Test
 module M1 =

    type System.Double with
        static member doit (v:double) = v


    let inline duck (v : ^a) = (^a : (static member doit: ^a -> ^a) (v))

    let (v:System.Double) = 2.8

    System.Double.doit(v)  // ok
    
   // type checker complains
    duck v

 module M2 =

    type A = CA
        with
          static member doit (v:A) = v  

    type B = CB

    // Both ok
    M1.duck CA
    A.doit CA        

 module M3 =

    type M2.B
        with
          static member doit (v:M2.B) = v  
    
    // type checker complains
    M1.duck M2.CB
    M2.B.doit (M2.CB)  // ok

Has anyone an explanation to this behavior?

Thank you

Regards

Jean-Christophe

Answer 1

Static member constraints only work when there are corresponding real .NET members on the type being used.  However, extension methods result in static methods on another type (even though you call them as if they were defined on the original type), so they can't be used with static member constraints.  In your examples, System.Double and M2.B don't really have "doit" methods so methods that statically require a "doit" method on those types won't work.