Outcome ODR

A.cpp

#include "A.hpp"

namespace A
{
    A::result foo()
    {
        return 5;
    }
}

A.hpp

#include <outcome.hpp>

namespace A
{
    namespace outcome = OUTCOME_V2_NAMESPACE;
    struct my_error {};
    using result = outcome::result<int, my_error>;

    A::result foo();
}

B.cpp

#include "B.hpp"
#include "A.hpp"

OUTCOME_V2_NAMESPACE_BEGIN
namespace convert
{
  template <>
  struct value_or_error<::B::result, ::A::result>
  {
    // True to indicate that this converter wants `result`/`outcome` to NOT reject all other `result`
    static constexpr bool enable_result_inputs = true;
    // False to indicate that this converter wants `outcome` to NOT reject all other `outcome`
    static constexpr bool enable_outcome_inputs = false;

    ::B::result operator()(::A::result &&src)
    {
        return 5;
    }
  };
}
OUTCOME_V2_NAMESPACE_END

namespace B
{
    result foo()
    {
        return B::result(A::foo());
    }
}

B.hpp

#include <outcome.hpp>

namespace B
{
    namespace outcome = OUTCOME_V2_NAMESPACE;
    using result = outcome::result<int>;

    B::result foo();
}

C.cpp

#include "C.hpp"
#include "A.hpp"

OUTCOME_V2_NAMESPACE_BEGIN
namespace convert
{
  template <>
  struct value_or_error<::C::result, ::A::result>
  {
    // True to indicate that this converter wants `result`/`outcome` to NOT reject all other `result`
    static constexpr bool enable_result_inputs = true;
    // False to indicate that this converter wants `outcome` to NOT reject all other `outcome`
    static constexpr bool enable_outcome_inputs = false;

    ::C::result operator()(::A::result &&src)
    {
        return 42;
    }
  };
}
OUTCOME_V2_NAMESPACE_END

namespace C
{
    result foo()
    {
        return C::result(A::foo());
    }
}

C.hpp

#include <outcome.hpp>

namespace C
{
    namespace outcome = OUTCOME_V2_NAMESPACE;
    using result = outcome::result<int>;

    C::result foo();
}

main.cpp

#include "B.hpp"
#include "C.hpp"

#include <iostream>

int main()
{
    {
        auto r = B::foo();
        std::cout << r.value() << '\n';
    }
    {
        auto r = C::foo();
        std::cout << r.value() << '\n';
    }
}