[Issue 22184] New: std.sumtype.SumType with mutual recursion causes a forward reference of variable `isHashable`

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=22184

          Issue ID: 22184
           Summary: std.sumtype.SumType with mutual recursion causes a
                    forward reference of variable `isHashable`
           Product: D
           Version: D2
          Hardware: x86
                OS: Linux
            Status: NEW
          Severity: regression
          Priority: P1
         Component: phobos
          Assignee: nobody at puremagic.com
          Reporter: ttanjo at gmail.com

The following code works with dmd 2.096.1 (uses sumtype via dub) but does not
work since dmd 2.097.0 (uses sumtype or std.sumtype).

I'm not sure it is a regression of std.sumtype or dmd. I tentatively set the
`Component` field to `phobos`.

run.dang.io
- (sumtype):     https://run.dlang.io/is/IiGESU
- (std.sumtype): https://run.dlang.io/is/j8rqve

```dlang
import std.sumtype;

class A
{
    SumType!B type;
}

class B
{
    SumType!C fields;
}

class C
{
    SumType!(B, int) type;
}

```

Error message:
```console
/dlang/dmd/linux/bin64/../../src/druntime/import/core/internal/traits.d(191):
Error: forward reference of variable `isHashable`
/dlang/dmd/linux/bin64/../../src/phobos/std/meta.d(842): Error: template
instance `core.internal.traits.allSat!(isHashable, const(B), const(int))` error
instantiating
/dlang/dmd/linux/bin64/../../src/phobos/std/sumtype.d(771):        instantiated
from here: `allSatisfy!(isHashable, const(B), const(int))`
onlineapp.d(15):        instantiated from here: `SumType!(B, int)`
/dlang/dmd/linux/bin64/../../src/druntime/import/core/internal/traits.d(191):
Error: template instance `std.sumtype.pred!(const(C))` error instantiating
/dlang/dmd/linux/bin64/../../src/phobos/std/meta.d(842):        instantiated
from here: `allSat!(isHashable, const(C))`
/dlang/dmd/linux/bin64/../../src/phobos/std/sumtype.d(771):        instantiated
from here: `allSatisfy!(isHashable, const(C))`
onlineapp.d(10):        instantiated from here: `SumType!(C)`
/dlang/dmd/linux/bin64/../../src/druntime/import/core/internal/traits.d(191):
Error: template instance `std.sumtype.pred!(const(B))` error instantiating
/dlang/dmd/linux/bin64/../../src/phobos/std/meta.d(842):        instantiated
from here: `allSat!(isHashable, const(B))`
/dlang/dmd/linux/bin64/../../src/phobos/std/sumtype.d(771):        instantiated
from here: `allSatisfy!(isHashable, const(B))`
onlineapp.d(5):        instantiated from here: `SumType!(B)`
```

Notes:
- It works if `A` is not available
- It works if `C` has a field whose type is `SumType!B` rather than
`SumType!(B, int)`

--