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

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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 at nondot.org
          Reporter: dermojo at gmail.com
                CC: blitzrakete at gmail.com, erik.pilkington at gmail.com,
                    llvm-bugs at lists.llvm.org, richard-llvm at 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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