'[llvm-bugs] [Bug 47356] New: c++2a reports an error of overloaded operator while c++11/17 accepts'

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List:       llvm-bugs
Subject:    [llvm-bugs] [Bug 47356] New: c++2a reports an error of overloaded operator while c++11/17 accepts
From:       via llvm-bugs <llvm-bugs () lists ! llvm ! org>
Date:       2020-08-30 13:39:24
Message-ID: bug-47356-206 () http ! bugs ! llvm ! org/
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--1598794764.b13EFf44.4969
Date: Sun, 30 Aug 2020 06:39:24 -0700
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https://bugs.llvm.org/show_bug.cgi?id=47356

            Bug ID: 47356
           Summary: c++2a reports an error of overloaded operator while
                    c++11/17 accepts
           Product: clang
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: C++2a
          Assignee: unassignedclangbugs@nondot.org
          Reporter: tangyixuan@mail.dlut.edu.cn
                CC: blitzrakete@gmail.com, erik.pilkington@gmail.com,
                    llvm-bugs@lists.llvm.org, richard-llvm@metafoo.co.uk

The following code is rejected by -std=c++2a, while ok with -std=c++11 or
-std=c++17.

$ cat s.cpp
template<typename T>
struct A {
  A() {}

  template<typename U>
    A(const A<U>&) {}

  bool operator==(const A&) const { return true; }
};

A<const int> a;
A<int> b;
auto c = (a == b);

$ clang -v
clang version 11.0.0 (https://github.com/llvm/llvm-project.git
ab4627364aefcaac751e757b721b7d07b48a545a)
Target: x86_64-unknown-linux-gnu
Thread model: posix

$ clang++ -c -std=c++17 s.cpp
success.

$ clang++ -c -std=c++2a s.cpp
s.cpp:13:13: error: use of overloaded operator '==' is ambiguous (with operand
types 'A<const int>' and 'A<int>')
auto c = (a == b);
          ~ ^  ~
s.cpp:8:8: note: candidate function
  bool operator==(const A&) const { return true; }
       ^
s.cpp:8:8: note: candidate function (with reversed parameter order)
1 error generated.

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--1598794764.b13EFf44.4969
Date: Sun, 30 Aug 2020 06:39:24 -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 - c++2a reports an error of overloaded operator while c++11/17 accepts"
   href="https://bugs.llvm.org/show_bug.cgi?id=47356">47356</a>
          </td>
        </tr>

        <tr>
          <th>Summary</th>
          <td>c++2a reports an error of overloaded operator while c++11/17 accepts
          </td>
        </tr>

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

        <tr>
          <th>Version</th>
          <td>trunk
          </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>enhancement
          </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>tangyixuan&#64;mail.dlut.edu.cn
          </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>The following code is rejected by -std=c++2a, while ok with -std=c++11 \
                or
-std=c++17.

$ cat s.cpp
template&lt;typename T&gt;
struct A {
  A() {}

  template&lt;typename U&gt;
    A(const A&lt;U&gt;&amp;) {}

  bool operator==(const A&amp;) const { return true; }
};

A&lt;const int&gt; a;
A&lt;int&gt; b;
auto c = (a == b);

$ clang -v
clang version 11.0.0 (<a \
href="https://github.com/llvm/llvm-project.git">https://github.com/llvm/llvm-project.git</a>
 ab4627364aefcaac751e757b721b7d07b48a545a)
Target: x86_64-unknown-linux-gnu
Thread model: posix

$ clang++ -c -std=c++17 s.cpp
success.

$ clang++ -c -std=c++2a s.cpp
s.cpp:13:13: error: use of overloaded operator '==' is ambiguous (with operand
types 'A&lt;const int&gt;' and 'A&lt;int&gt;')
auto c = (a == b);
          ~ ^  ~
s.cpp:8:8: note: candidate function
  bool operator==(const A&amp;) const { return true; }
       ^
s.cpp:8:8: note: candidate function (with reversed parameter order)
1 error generated.</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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