'[llvm-bugs] [Bug 46508] New: use of overloaded operator '==' is ambiguous (with reversed parameter o'

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List:       llvm-bugs
Subject:    [llvm-bugs] [Bug 46508] New: use of overloaded operator '==' is ambiguous (with reversed parameter o
From:       via llvm-bugs <llvm-bugs () lists ! llvm ! org>
Date:       2020-06-30 6:22:31
Message-ID: bug-46508-206 () http ! bugs ! llvm ! org/
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Date: Mon, 29 Jun 2020 23:22:31 -0700
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https://bugs.llvm.org/show_bug.cgi?id=46508

            Bug ID: 46508
           Summary: use of overloaded operator '==' is ambiguous (with
                    reversed parameter order)
           Product: clang
           Version: 10.0
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: C++2a
          Assignee: unassignedclangbugs@nondot.org
          Reporter: dermojo@gmail.com
                CC: blitzrakete@gmail.com, erik.pilkington@gmail.com,
                    llvm-bugs@lists.llvm.org, richard-llvm@metafoo.co.uk

Hi,

The following code is a very reduced example from a template expression
library:

  class Foo {};

  class X {
  public:
      template <typename T>
      constexpr Foo operator==(const T&) const 
      {
        return Foo{};
      }
  };

  class A : public X {};
  class B : public X {};

  int main() {
    Foo f = A{} == B{};
    return 0;
  }

It fails to compile with C++20:
> clang++-10 -c sample.cpp -std=c++20
  sample.cpp:16:16: error: use of overloaded operator '==' is ambiguous (with
operand types 'A' and 'B')
     Foo f = A{} == B{};
             ~~~ ^  ~~~
  sample.cpp:6:18: note: candidate function [with T = B]
     constexpr Foo operator==(const T&) const
                 ^
  sample.cpp:6:18: note: candidate function (with reversed parameter order)
[with T = A]
  1 error generated.

I can't figure out why the reversed operator is used here (tbh, I'm not a
language lawyer).

Any help is appreciated, thanks!

> clang++-10 --version
clang version 10.0.0-4ubuntu1~18.04.1

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--1593498151.f0eEb67C3.24786
Date: Mon, 29 Jun 2020 23:22:31 -0700
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<html>
    <head>
      <base href="https://bugs.llvm.org/">
    </head>
    <body><table border="1" cellspacing="0" cellpadding="8">
        <tr>
          <th>Bug ID</th>
          <td><a class="bz_bug_link 
          bz_status_NEW "
   title="NEW - use of overloaded operator '==' is ambiguous (with reversed parameter \
order)"  href="https://bugs.llvm.org/show_bug.cgi?id=46508">46508</a>
          </td>
        </tr>

        <tr>
          <th>Summary</th>
          <td>use of overloaded operator '==' is ambiguous (with reversed parameter \
order)  </td>
        </tr>

        <tr>
          <th>Product</th>
          <td>clang
          </td>
        </tr>

        <tr>
          <th>Version</th>
          <td>10.0
          </td>
        </tr>

        <tr>
          <th>Hardware</th>
          <td>PC
          </td>
        </tr>

        <tr>
          <th>OS</th>
          <td>Linux
          </td>
        </tr>

        <tr>
          <th>Status</th>
          <td>NEW
          </td>
        </tr>

        <tr>
          <th>Severity</th>
          <td>normal
          </td>
        </tr>

        <tr>
          <th>Priority</th>
          <td>P
          </td>
        </tr>

        <tr>
          <th>Component</th>
          <td>C++2a
          </td>
        </tr>

        <tr>
          <th>Assignee</th>
          <td>unassignedclangbugs&#64;nondot.org
          </td>
        </tr>

        <tr>
          <th>Reporter</th>
          <td>dermojo&#64;gmail.com
          </td>
        </tr>

        <tr>
          <th>CC</th>
          <td>blitzrakete&#64;gmail.com, erik.pilkington&#64;gmail.com, \
llvm-bugs&#64;lists.llvm.org, richard-llvm&#64;metafoo.co.uk  </td>
        </tr></table>
      <p>
        <div>
        <pre>Hi,

The following code is a very reduced example from a template expression
library:

  class Foo {};

  class X {
  public:
      template &lt;typename T&gt;
      constexpr Foo operator==(const T&amp;) const 
      {
        return Foo{};
      }
  };

  class A : public X {};
  class B : public X {};

  int main() {
    Foo f = A{} == B{};
    return 0;
  }

It fails to compile with C++20:
<span class="quote">&gt; clang++-10 -c sample.cpp -std=c++20</span >
  sample.cpp:16:16: error: use of overloaded operator '==' is ambiguous (with
operand types 'A' and 'B')
     Foo f = A{} == B{};
             ~~~ ^  ~~~
  sample.cpp:6:18: note: candidate function [with T = B]
     constexpr Foo operator==(const T&amp;) const
                 ^
  sample.cpp:6:18: note: candidate function (with reversed parameter order)
[with T = A]
  1 error generated.

I can't figure out why the reversed operator is used here (tbh, I'm not a
language lawyer).

Any help is appreciated, thanks!

<span class="quote">&gt; clang++-10 --version</span >
clang version 10.0.0-4ubuntu1~18.04.1</pre>
        </div>
      </p>

      <hr>
      <span>You are receiving this mail because:</span>

      <ul>
          <li>You are on the CC list for the bug.</li>
      </ul>
    </body>
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