[Issue 22010] Link error with mutually recursive SumType / struct with opEquals

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=22010

Mathias LANG <pro.mathias.lang at gmail.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |pro.mathias.lang at gmail.com

--- Comment #4 from Mathias LANG <pro.mathias.lang at gmail.com> ---
I believe I hit this issue as well. The key seems to be the mutual recursion. 

In my case, the code is the following:
```
import std.sumtype;
import std.stdio;

alias ST = SumType!(int, long, Custom[]);

struct Custom
{
    int a;
    ST b;
}

void main ()
{
    const SumType!(int, long, Custom[]) st = [ Custom(42), Custom(69) ];
    version(all) writeln(st);
    assert(st.toString() == "NULL");
}
```

With this, I get a compiler error:
```
/usr/include/dmd/phobos/std/sumtype.d(1945): Error: need `this` for `tags` of
type `ulong[1]`
Error: `this` for `__invariant1322` needs to be type `TagTuple` not type
`const(TagTuple)`
/usr/include/dmd/phobos/std/sumtype.d(2102): Error: static assert:  "No
matching handler for types `(const(Custom[]))`"
/usr/include/dmd/phobos/std/sumtype.d(1662):        instantiated from here:
`matchImpl!(const(SumType!(int, long, Custom[])))`
/usr/include/dmd/phobos/std/sumtype.d(787):        instantiated from here:
`match!(const(SumType!(int, long, Custom[])))`
st.d(16):        instantiated from here: `toString!(const(SumType!(int, long,
Custom[])))`
```

Now if you remove the `writeln` in `version(all)`, you end up with:
```
ld: error: undefined symbol:
_D3std6format5write__T11formatValueTSQBj5array__T8AppenderTAyaZQoTAxS2st6CustomTaZQCiFKQBzKQzMKxSQDrQDq4spec__T10FormatSpecTaZQpZv
>>> referenced by st.d
>>>               st.o:(_D3std4conv__T5toStrTAyaTAxS2st6CustomZQzFQrZQy)
collect2: error: ld returned 1 exit status
Error: linker exited with status 1
```

Looking at SumType's code, it seems the instantiation of `formatValue` happens
here:
https://github.com/dlang/phobos/blob/dba1bbe271a9b2d7f24edeebbc77846e29904e41/std/sumtype.d#L807

--