On Monday, 17 May 2021 at 14:35:51 UTC, Steven Schveighoffer
wrote:
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
The dual context that warning is referring to is vthis2, which
the gdc maintainer struggle to implemented for the gdc compiler.
A correct fix involves templates having their own context.
-Alex
