inout delegate

[Nicholas Wilson via Digitalmars-d]

On Sunday, 2 October 2016 at 09:55:26 UTC, Manu wrote:
> Can someone explain this to me?
>
> class Test
> {
>   inout(int) f() inout { return 10; }
>
>   void t()
>   {
>     f(); // calls fine with mutable 'this'
>     auto d = &this.f; // error : inout method Test.f is not 
> callable
> using a mutable this
>     d();
>   }
> }
>
> That error message seems very unhelpful, and it's not true. Of 
> course an inout method is callable with a mutable 'this'...
>
> I suspect that the problem is the type for the delegate; 
> "inout(int)
> delegate()" doesn't leave anything for the type system to 
> resolve the
> inout with.
> I guess the expectation is that this delegate has it's 
> inout-ness
> resolved when you capture the delegate:
>   is(typeof(&this.f) == int delegate())
> Or if 'this' were const:
>   is(typeof(&this.f) == const(int) delegate())
>
> But I get this unhelpful error instead.
>
> What's the story here?

That doesn't compile for me (using ad4a81b), but I get a 
different error message with an accompanying main
void main()
{
	const ct = new Test();
     ct.t();
     auto at = new Test();
     at.t();
     immutable it = new Test();
     it.t();
}

Error: mutable method Test.t is not callable using a 
(const|immutable) object
Note t, not f.

Making t inout fixes this.