GoogleTest fails to use custom comparison operator

Question

I wanted to do simple tests on my Word struct in Google Test. To make the test code simpler and shorter I decided to write a comparison operator (even though I don't really need one) and just use ASSERT_EQ, as per the primer. Even though everything seems OK, I get a compiler error:

/* Word.h */
namespace tgs {

struct Word {
    //something here
    Word();
    virtual ~Word();
    bool operator== (Word& rhs);
};
}

/* Word.cpp */
namespace tgs {
bool Word::operator== (Word& rhs){
  return true; //there are actual member comparisons here
}
}


/* TextAreaShould_test.cc */
TEST_F(TextAreaShould, DoSomething) {
  Word w1, w2;

  if(w1 == w2){ //compiles and runs ok
    0;
  }
  ASSERT_EQ(w1, w2); //produces following error
}

compiler error:

[ 86%] Building CXX object CMakeFiles/tests/TextAreaShould.test.dir/tests/TextAreaShould_test.cc.o
/usr/bin/c++    -I/home/nietaki/zpp/TheGameShow_build -I/home/nietaki/zpp/TheGameShow -I/home/nietaki/zpp/TheGameShow/google_mock/include -I/home/nietaki/zpp/TheGameShow/google_mock/gtest/include -I/usr/local/include   -o CMakeFiles/tests/TextAreaShould.test.dir/tests/TextAreaShould_test.cc.o -c /home/nietaki/zpp/TheGameShow/tests/TextAreaShould_test.cc
In file included from /home/nietaki/zpp/TheGameShow/tests/TextAreaShould_test.cc:8:
/home/nietaki/zpp/TheGameShow/google_mock/gtest/include/gtest/gtest.h: In function ‘testing::AssertionResult testing::internal::CmpHelperEQ(const char*, const char*, const T1&, const T2&) [with T1 = tgs::Word, T2 = tgs::Word]’:
/    home/nietaki/zpp/TheGameShow/google_mock/gtest/include/gtest/gtest.h:1363:   instantiated from ‘static testing::AssertionResult testing::internal::EqHelper<lhs_is_null_literal>::Compare(const char*, const char*, const T1&, const T2&) [with T1 = tgs::Word, T2 = tgs::Word, bool lhs_is_null_literal = false]’
/home/nietaki/zpp/TheGameShow/tests/TextAreaShould_test.cc:112:   instantiated from here
/home/nietaki/zpp/TheGameShow/google_mock/gtest/include/gtest/gtest.h:1326: error: no match for ‘operator==’ in ‘expected == actual’
/home/nietaki/zpp/TheGameShow/renderer/textarea/Word.h:25: note: candidates are: bool tgs::Word::operator==(tgs::Word&)

I also tried placing the comparison operator outside the class, not as a member, but it doesn't seem to make much of a difference. I tried to do everything according to the Primer, but perhaps something is wrong.

Answer 1

The comparison operator doesn't change any of the arguments so it can be const, as used by Gooogle Test:

struct Word {

    Word();
    virtual ~Word();
    bool operator== (const Word& rhs) const;
};

...and everything works out allright.