Boost.Decimal issue 1260

[mborland]

#define BOOST_DECIMAL_DETAIL_INT128_NO_CONSTEVAL_DETECTION

#include <boost/decimal.hpp>
#include <boost/core/lightweight_test.hpp>

template <typename T>
void test()
{
    const T lhs {"1E34"};
    const T rhs {"-0.51"};
    const T res {"9999999999999999999999999999999999"};

    const T add_val {lhs + rhs};
    BOOST_TEST_EQ(add_val, res);

    const T sub_val {lhs - boost::decimal::abs(rhs)};
    BOOST_TEST_EQ(sub_val, res);
}

int main()
{
    // TODO(mborland): GCC 12+ in 32 bit compilations only with release mode fail
    #if 1

    test<boost::decimal::decimal128_t>();
    test<boost::decimal::decimal_fast128_t>();

    #endif

    return boost::report_errors();
}

If we define no consteval detection the result becomes correct, without its:

/home/mborland/Documents/boost/libs/decimal/test/github_issue_1260.cpp(20): test 'add_val == res' ('9999999999999999999999999999999990' == '9999999999999999999999999999999999') failed in function 'void test() [with T = boost::decimal::decimal128_t]'
/home/mborland/Documents/boost/libs/decimal/test/github_issue_1260.cpp(23): test 'sub_val == res' ('9999999999999999999999999999999990' == '9999999999999999999999999999999999') failed in function 'void test() [with T = boost::decimal::decimal128_t]'
/home/mborland/Documents/boost/libs/decimal/test/github_issue_1260.cpp(20): test 'add_val == res' ('9999999999999999999999999999999990' == '9999999999999999999999999999999999') failed in function 'void test() [with T = boost::decimal::decimal_fast128_t]'
/home/mborland/Documents/boost/libs/decimal/test/github_issue_1260.cpp(23): test 'sub_val == res' ('9999999999999999999999999999999990' == '9999999999999999999999999999999999') failed in function 'void test() [with T = boost::decimal::decimal_fast128_t]'
4 errors detected.