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From: "de34 at live dot cn" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug libstdc++/111358] New: libstdc++'s optional::and_then and
 optional::transform are not ADL-proof
Date: Sun, 10 Sep 2023 08:18:32 +0000
X-Bugzilla-Reason: CC
X-Bugzilla-Type: new
X-Bugzilla-Watch-Reason: None
X-Bugzilla-Product: gcc
X-Bugzilla-Component: libstdc++
X-Bugzilla-Version: 14.0
X-Bugzilla-Keywords: rejects-valid
X-Bugzilla-Severity: normal
X-Bugzilla-Who: de34 at live dot cn
X-Bugzilla-Status: UNCONFIRMED
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X-Bugzilla-Priority: P3
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D111358

            Bug ID: 111358
           Summary: libstdc++'s optional::and_then and optional::transform
                    are not ADL-proof
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Keywords: rejects-valid
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: de34 at live dot cn
  Target Milestone: ---

The following program is incorrectly rejected when using libstdc++
(https://godbolt.org/z/eavTYjEdx):
```
#include <optional>

namespace fvs {
    struct Foo {};

    void operator*(std::optional<fvs::Foo>) =3D delete;
}

int main()
{
    std::optional<fvs::Foo>{}.and_then(
        [](auto&&){ return std::optional<char>{}; });
    std::optional<fvs::Foo>{}.transform(
        [](auto&&){ return 0; });
}
```

The reason of compilation error is that libstdc++ uses **this in and_then a=
nd
transform functions, which triggers ADL and finds unwanted operator*().

These monadic operations are currently specified in [optional.monadic] with
value(), so they should be ADL-proof.=