• Andrey Zherikov (27/27) Nov 28 2022 The following code doesn't compile:
• Andrey Zherikov (9/9) Nov 28 2022 If I replace `Nullable` with `SumType!(None,S)` then everything
• Steven Schveighoffer (11/21) Nov 28 2022 It's just a forward reference bug.

Andrey Zherikov <andrey.zherikov gmail.com> writes:

The following code doesn't compile:
```d
struct S
{
     Nullable!S delegate() f;
}
```
Output from DMD:
```
/dlang/dmd/linux/bin64/../../src/druntime/import/core/internal/traits.d(345):
Error: unable to determine fields of `S` because of forward references
/dlang/dmd/linux/bin64/../../src/phobos/std/traits.d(3327): 
Error: template instance 
`core.internal.traits._hasIndirections!(S)` error instantiating
/dlang/dmd/linux/bin64/../../src/phobos/std/typecons.d(2768):     
    instantiated from here: `hasIndirections!(S)`
onlineapp.d(4):        instantiated from here: `Nullable!(S)`
/dlang/dmd/linux/bin64/../../src/druntime/import/core/internal/traits.d(16):
Error: unable to determine fields of `S` because of forward references
/dlang/dmd/linux/bin64/../../src/druntime/import/core/internal/traits.d(269):
Error: template instance `core.internal.traits.Fields!(S)` error instantiating
/dlang/dmd/linux/bin64/../../src/phobos/std/typecons.d(2790):     
    instantiated from here: `hasElaborateDestructor!(S)`
onlineapp.d(4):        instantiated from here: `Nullable!(S)`
```
I'm not sure whether it's a big in Nullable or in compiler 
because `pragma(msg, S.sizeof)` doesn't work either.

I tried different variants like `function` instead of `delegate` 
and putting `Nullable` to parameters like `void 
function(Nullable!S) f;` - but they don't work either.

Andrey Zherikov <andrey.zherikov gmail.com> writes:

If I replace `Nullable` with `SumType!(None,S)` then everything 
works:
```d
struct None {}
struct S
{
     SumType!(None,S) delegate() f;
}
```

Steven Schveighoffer <schveiguy gmail.com> writes:

On 11/28/22 6:07 AM, Andrey Zherikov wrote:
 The following code doesn't compile:
 ```d
 struct S
 {
      Nullable!S delegate() f;
 }
 ```

 I tried different variants like `function` instead of `delegate` and 
 putting `Nullable` to parameters like `void function(Nullable!S) f;` - 
 but they don't work either.

It's just a forward reference bug.

This seems to fix it:

```d
alias NS = Nullable!S;
struct S
{
    Nullable!S delegate() f;
}
```

-Steve