Different behavior in candidate selection between GCC 11 and GCC 12

I'm encountering a different behavior when compiling the following code with GCC 11 and GCC 12 on Ubuntu 22.04:

#include <iostream>#include <algorithm>namespace Foo {  class MyClass {  private:    int a;  public:    int getA() const { return a; }  };}bool operator==(const Foo::MyClass &lhs, const Foo::MyClass &rhs) {  return lhs.getA() == rhs.getA();}int main() {  Foo::MyClass myClass1;  Foo::MyClass myClass2;  Foo::MyClass *p1 = &myClass1;  Foo::MyClass *p2 = &myClass2;  bool equal = std::equal(p1, p1 + 1, p2);  std::cout << "equal: " << equal << std::endl;  return 0;}

The detail compilation command (from cmake's verbose output) is:

/usr/bin/c++   -std=gnu++11 -MD -MT CMakeFiles/cmake-lab.dir/src/main.cpp.o -MF CMakeFiles/cmake-lab.dir/src/main.cpp.o.d -o CMakeFiles/cmake-lab.dir/src/main.cpp.o -c /home/wbhu/dev/CodeShare/cpp-proj/cmake-lab/src/main.cpp

When compiling with GCC 11, I get the following error:

/home/wbhu/dev/CodeShare/cpp-proj/cmake-lab/src/main.cpp:30:26:   required from here/usr/include/c++/11/bits/stl_algobase.h:1162:29: error: no match for ‘operator==’ (operand types are ‘Foo::MyClass’ and ‘Foo::MyClass’) 1162 |             if (!(*__first1 == *__first2))      |                  ~~~~~~~~~~~^~~~~~~~~~~~~

It seems that GCC 11 cannot find the operatior== defined outside of namespace Foo. However, the code compiles without any issues in GCC 12.

Interestingly, if I move MyClass out of the Foo namespace or move operator== inside the Foo namespace, the code compiles correctly in both GCC 11 and GCC 12

My questions are:

• Which compiler's behavior is correct here?
• What is causing this difference in behavior between GCC 11 and GCC 12?

Any insights or clarifications would be greatly appreciated!