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

João Lourenço <jlourenco5691@gmail.com> changed:

           What    |Removed                     |Added
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                 CC|                            |jlourenco5691@gmail.com

--- Comment #1 from João Lourenço <jlourenco5691@gmail.com> ---
I think this is because it uses `match` internally. You are passing a `const Ref!T` in your `opEquals` which makes its `TagTuple` a `const(TagTuple)`. When trying to match it won't be able to.

As a workaround fix, removing `const` from `opEquals` solves your issue.

Also, refactoring your `opEquals` to a template (to allow the program to compile), gives a more propper error when comparing the types:

```d
/+dub.sdl:
dependency "sumtype" version="~>1.1.1"
+/
import sumtype;

struct S { Ref!Node node; }
alias Node = SumType!S;

private struct Ref(T)
{
    private T* _ref_ptr;
    this(ref T value) { _ref_ptr = &value; } ///
    bool opEquals()(ref const Ref!T other) { return *_ref_ptr ==
*other._ref_ptr; }
}

void main()
{
    S s;
    assert(S.init == s);
}
```

Produces (tested with sumtype-1.1.1):
---
.dub/packages/sumtype-1.1.1/sumtype/src/sumtype.d(1718,13): Error: need `this`
for `tags` of type `ulong[2]`
.dub/packages/sumtype-1.1.1/sumtype/src/sumtype.d(1718,13): Error: need `this`
for `tags` of type `ulong[2]`
Error: `this` for `__invariant430` needs to be type `TagTuple` not type
`const(TagTuple)`
.dub/packages/sumtype-1.1.1/sumtype/src/sumtype.d(1828,4): Error: static
assert:  "`handlers[0]` of type `template` never matches"
.dub/packages/sumtype-1.1.1/sumtype/src/sumtype.d(1448,46):        instantiated
from here: `matchImpl!(const(SumType!(S)), const(SumType!(S)))`
.dub/packages/sumtype-1.1.1/sumtype/src/sumtype.d(578,31):        instantiated
from here: `match!(const(SumType!(S)), const(SumType!(S)))`
.dub/packages/sumtype-1.1.1/sumtype/src/sumtype.d(587,11):        instantiated
from here: `opEquals!(const(SumType!(S)), const(SumType!(S)))`
onlineapp.d(14,53):        instantiated from here: `opEquals!(SumType!(S),
const(SumType!(S)))`
onlineapp.d(10,9):        instantiated from here: `opEquals!()`
dmd failed with exit code 1.
---

This should work though.

--

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

--- Comment #2 from Vladimir Panteleev <dlang-bugzilla@thecybershadow.net> ---
(In reply to João Lourenço from comment #1)
> I think this is because it uses `match` internally.

A link error like this should never happen no matter what the code does (unless it's something intentional like an extern declaration with no definition, of course.)

> As a workaround fix, removing `const` from `opEquals` solves your issue.

Thanks but this is a reduced example meant to illustrate the compiler bug.

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

--- Comment #3 from João Lourenço <jlourenco5691@gmail.com> ---
(In reply to Vladimir Panteleev from comment #2)
> Thanks but this is a reduced example meant to illustrate the compiler bug.

Yes, I mislead the issue and thought it was about SumType itself.

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

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

           What    |Removed                     |Added
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                 CC|                            |pro.mathias.lang@gmail.com

--- Comment #4 from Mathias LANG <pro.mathias.lang@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

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

Iain Buclaw <ibuclaw@gdcproject.org> changed:

           What    |Removed                     |Added
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           Priority|P1                          |P3

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