Address of UFCS call implicity converts to Delegate

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On Monday, 24 April 2017 at 15:47:14 UTC, Jonathan Marler wrote:
> I've added a DIP for this 
> (https://github.com/dlang/DIPs/pull/61).
>
> At first I first thought that all we needed was to add 
> semantics to take the address of a UFCS-style call, but after 
> messing around with your example I realized that delegates are 
> not ABI-compatible with functions that take the delegate ptr as 
> the first parameter.  You mentioned that the problem was with 
> the parameter order and that this should work with extern(C) 
> functions and I think you're right.
>
> The new DIP proposes the addition of "Extension Methods" which 
> are functions that are ABI-compatible with delegates. You 
> define an extension method by naming the first parameter "this":
>
> struct Foo
> {
>     void bar(int x)
>     {
>     }
> }
> void baz(ref Foo this, int x)
> {
> }
>
> Because the first parameter of baz is named this, it is an 
> "extension method" of Foo which means it is ABI-compatible with 
> the method bar.
>
> void delegate(int x) dg;
> Foo foo;
>
> dg = &foo.bar;  // a normal method delegate
> dg(42);         // calls foo.bar(42)
>
> dg = &foo.baz;  // an extension method delegate
> dg(42);         // calls baz(foo, 42);
>
> dg = &baz;      // a "null delegate", unsafe code, funcptr 
> points to the baz function, but ptr is null
> dg(42);         // calls baz(null, 42);

One small tweak is that `this` should act as a storage class 
instead of the user having to name the parameter `this`. This is 
what C# does so we should mimic it to avoid confusion.

https://www.codeproject.com/Tips/709310/Extension-Method-In-Csharp