dual-context deprecation

[Steven Schveighoffer]

On 5/17/21 9:47 AM, jmh530 wrote:
> The code below (simplified from my actual problem) generates a warning 
> that member function b "requires a dual-context, which is deprecated".
> 
> However when I look at the list of deprecated features [1], I'm not 
> seeing which one this is referring to. Is it a valid deprecation?
> 
> I could only find this [2] reference to dual-contexts, which suggests 
> that the problem relates to passing aliases into member functions. 
> Moving it to a member function fixes the problem. Alternately, I could 
> make the alias part of Foo's type. My use case it is just a little 
> easier structured like this, but I get that there are workarounds.
> 
> My bigger question is about why it isn't listed more than anything. 
> I.e., should I file something in bugzilla.
> 
> ```d
> struct Foo
> {
>      double a;
> 
>      this(double x)
>      {
>          this.a = x;
>      }
> 
>      double b(alias inverse)()
>      {
>          return inverse(a);
>      }
> }
> 
> void main()
> {
>      auto foo = Foo(2.0);
>      auto x = foo.b!(a => (10.0 ^^ a))();
> }
> ```

The feature is deprecated in its current form. The issue as I understand 
it (i.e. very little) is that compilers other than DMD could not use 
this same way to implement dual contexts, and so they could not have the 
feature. This means that valid code in DMD would not compile on GDC or LDC.

The way forward was to deprecate the mechanism used to implement it for 
DMD, and at some point tackle it in a backend-agnostic way.

Personally, I don't know why we can't fix it so that it's portable, but 
I understand so little about compilers that I've pretty much stayed out 
of it. The feature is very much needed.

-Steve