Recursive delegate inside template?

[drathier]

On Friday, 26 June 2020 at 15:10:07 UTC, Adam D. Ruppe wrote:
> On Friday, 26 June 2020 at 14:12:00 UTC, drathier wrote:
>> I'm trying to get this to compile, without much luck:
>
> You might be able to make it a function:
>
>
> bool foo()
> {
>     auto fn(A)()
>     {
>         A delegate(A) fn;
>         fn = delegate A(A a)
>         {
>             return fn(a);
>         };
>         return fn;
>     }
>
>     return fn!(bool)()(true);
> }
>

I tried this, but it gives me `Error: non-constant expression 
&fn` which I don't understand. Is ctfe kicking in even though 
this is inside a function?

```
void foo2()
{
     auto fn(A) = std.functional.toDelegate(({
             A fn(A)(A a)
             {
                 return a;
             }

             return &fn!(A);
         }()));
}
```

>> but I need the function to be polymorphic, so I need the 
>> template.
>
> I don't really understand your need though, can you post more 
> context?

I'm generating code, compiling a strict pure functional language 
to D. The code that I'm trying to compile is this example 
function:
```
fn a =
     let
         _ = fn a
     in a
```
which gets the type `forall a. a -> a` inferred.

That's something like
```
A delegate(A) fn(A) = delegate A (A a)
{
     auto _ = fn!(A)(a);
     return a;
};
```
in D syntax, if it would compile. Well, it compiles, but the 
linker fails `error: TLS relocation against invalid instruction`.

It's a simplified example that still tests these features:
- recursion
- polymorphism
- functions as values (closures/delegates)