Better Google Test Support for std::optional.

opt-gtest.h

// std gtest comparators
// Copyright 2022 Sean Farrell <sean.farrell@rioki.org>
//
// This program is free software. It comes without any warranty, to
// the extent permitted by applicable law. You can redistribute it
// and/or modify it under the terms of the Do What The Fuck You Want
// To Public License, Version 2, as published by Sam Hocevar. See
// http://www.wtfpl.net/ for more details.

#pragma once

#include <string>
#include <iostream>
#include <sstream>

namespace std
{
    template <typename T>
    std::ostream& operator << (std::ostream& os, const std::optional<T>& v)
    {
        if (v)
        {
            os << *v;
        }
        else
        {
            os << "nullopt";
        }
        return os;
    }
}

namespace std::test
{
    template <typename T>
    bool is_nullopt(const std::optional<T>& v)
    {
        return v == std::nullopt;
    }

    template <typename T>
    bool is_nullopt(const T& v)
    {
        return false;
    }

    template <typename T1, typename T2>
    testing::AssertionResult compare_optional(const char* lhs_expression, const char* rhs_expression, const T1& lhs_value, const T2& rhs_value)
    {
        if (lhs_value == rhs_value)
        {
            return testing::AssertionSuccess();
        }

        if (is_nullopt(lhs_value))
        {
            return testing::AssertionFailure()
                << "The value " << lhs_expression << " is nullopt,"
                << " but the value " << rhs_expression << " has the value " << rhs_value << ".";
        }

        if (is_nullopt(rhs_value))
        {
            return testing::AssertionFailure()
                << "The value " << lhs_expression << " has the value " << lhs_value
                << " but the value " << rhs_expression << " is nullopt.";
        }

        ::std::stringstream lhs_ss;
        lhs_ss << lhs_value;

        ::std::stringstream rhs_ss;
        rhs_ss << rhs_value;

        return testing::internal::EqFailure(lhs_expression,
            rhs_expression,
            testing::internal::StringStreamToString(&lhs_ss),
            testing::internal::StringStreamToString(&rhs_ss),
            false);
    }
}

#define EXPECT_OPTIONAL_EQ(val1, val2) EXPECT_PRED_FORMAT2(::std::test::compare_optional, val1, val2)
#define ASSERT_OPTIONAL_EQ(val1, val2) ASSERT_PRED_FORMAT2(::std::test::compare_optional, val1, val2)