Can't use overloaded comparison operator with Catch test

Question

I have a simple unit-test using Catch 2.11.1:

#define CATCH_CONFIG_MAIN
#include "catch.hpp"
#include <utility>
#include <any>

namespace A::B
{
    namespace C
    {
        struct S
        {
        };
    }

    using type = std::pair<C::S, std::any>;
}

inline bool operator==(A::B::type const&, A::B::type const&)
{
    return true;
}

TEST_CASE("test", "[test]")
{
    auto t1 = std::make_pair(A::B::C::S(), std::any());
    auto t2 = std::make_pair(A::B::C::S(), std::any());

    REQUIRE(t1 == t2);
}

The above simple programs generates the following errors:

$ g++ -Wall -Wextra -Wpedantic test-single.cpp -std=c++17
In file included from /usr/include/c++/9/bits/stl_algobase.h:64,
                 from /usr/include/c++/9/bits/char_traits.h:39,
                 from /usr/include/c++/9/string:40,
                 from catch.hpp:457,
                 from test-single.cpp:2:
/usr/include/c++/9/bits/stl_pair.h: In instantiation of ‘constexpr bool std::operator==(const std::pair<_T1, _T2>&, const std::pair<_T1, _T2>&) [with _T1 = A::B::C::S; _T2 = std::any]’:
catch.hpp:2289:98:   required from ‘bool Catch::compareEqual(const LhsT&, const RhsT&) [with LhsT = std::pair<A::B::C::S, std::any>; RhsT = std::pair<A::B::C::S, std::any>]’
catch.hpp:2318:34:   required from ‘const Catch::BinaryExpr<LhsT, const RhsT&> Catch::ExprLhs<LhsT>::operator==(const RhsT&) [with RhsT = std::pair<A::B::C::S, std::any>; LhsT = const std::pair<A::B::C::S, std::any>&]’
test-single.cpp:28:5:   required from here
/usr/include/c++/9/bits/stl_pair.h:449:24: error: no match for ‘operator==’ (operand types are ‘const A::B::C::S’ and ‘const A::B::C::S’)
  449 |     { return __x.first == __y.first && __x.second == __y.second; }
      |              ~~~~~~~~~~^~~~~~~~~~~~

_{[And many many more messages after this...]}

The crucial part of the error message is this line:

/usr/include/c++/9/bits/stl_pair.h: In instantiation of ‘constexpr bool std::operator==(const std::pair<_T1, _T2>&, const std::pair<_T1, _T2>&) [with _T1 = A::B::C::S; _T2 = std::any]’:

From the error message it's clear that it's the standard std::operator== function for std::pair that is being invoked, instead of my overloaded operator== function.

If I don't do the comparison inside the Catch REQUIRE macro, then it works:

auto result = t1 == t2;  // Invokes my overloaded comparison operator
REQUIRE(result);

Now is this a problem with Catch, or with my operator function?

---

NB: I'm building on Debian SID with a recent build of GCC 9.2

$ g++ --version
g++ (Debian 9.2.1-23) 9.2.1 20200110
Copyright (C) 2019 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

Answer 1

Note that even with the parentheses suggested by Lightness, the code you show is exceptionally fragile.

I guess you are originally in ADL-only territory due to dependent name lookup inside the macro (see the last notes of https://en.cppreference.com/w/cpp/language/adl), and your code is quite clearly not ADL-viable. Adding parentheses makes the whole thing just an unqualified lookup, not ADL-only (again, a guess). The non-ADL part of unqualified lookup saves you in this case, but it will fall apart from entirely unrelated code changes.

Consider this code instead of the TEST_CASE, which is what using parentheses presumably boils down to:

namespace test
{
    bool foo()
    {
        auto t1 = std::make_pair(A::B::C::S(), std::any());
        auto t2 = std::make_pair(A::B::C::S(), std::any());

        return t1 == t2;
    }
}

This compiles and works as expected: https://godbolt.org/z/HiuWWy

Now add a completely unrelated operator== between your global operator== and the t1 == t2:

namespace test
{
    struct X{};
    bool operator==(X, X);

    bool foo()
    {
        auto t1 = std::make_pair(A::B::C::S(), std::any());
        auto t2 = std::make_pair(A::B::C::S(), std::any());

        return t1 == t2;
    }
}

And you're out for the count: https://godbolt.org/z/BUQC9Y

The operator== in the global namespace isn't found because the (non-ADL part of) unqualified name lookup stops in the first enclosing scope that has any operator==. Since that doesn't find anything useful, it falls back to using the inbuilt std::pair comparison operator (found via ADL), which won't work.

Just put operator overloads in the namespaces of the objects they operate on. And by corollary, don't overload operators for facilities from std (or other namespaces you are not allowed to touch).

---

Adding from comments:

The standard currently also says that the namespaces of template arguments are considered, so putting the operator== in namespace C would work (because the first template argument of std::pair comes from there): https://godbolt.org/z/eV8Joj

However, 1. that doesn't mesh too well with your type alias and 2. there is some movement to make ADL less wild and I've seen discussion to get rid of the "consider namespaces of template parameters". See http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p0934r0.pdf:

Why on earth would we look into the namespaces of template arguments? Nothing in there could possibly be part of the interface of the type, unless the template arguments were also base classes or something. -- Herb Sutter

I don't know where this paper stands today but I would avoid relying on this kind of ADL in new code.