Boost.Regex/ICU will no longer compile with clang 19

[pkl97]

This simple test program triggers a warning in clang 18 that indicates that the code will no longer compile with clang 19:

#include <iostream>
#include <boost/regex/icu.hpp>

int main()
{
    const boost::u32regex r1(boost::make_u32regex(".*"));
    const boost::u32regex r2(boost::make_u32regex(".*"));

    std::cout << "identical: " << (r1==r2?"yes":"no") << std::endl;
    return 0;
}

In file included from main.cpp:1:
In file included from include/c++/v1/iostream:43:
In file included from include/c++/v1/ios:221:
In file included from include/c++/v1/__fwd/ios.h:13:
include/c++/v1/__fwd/string.h:45:41: warning: 'char_traits' is deprecated: char_traits for T not equal to char, wchar_t, char8_t, char16_t or char32_t is non-standard and is provided for a temporary period. It will be removed in LLVM 19, so please migrate off of it. [-Wdeprecated-declarations]
45 | template <class _CharT, class _Traits = char_traits<_CharT>, class _Allocator = allocator<_CharT> >
| ^
Boost/1.83.0/include/boost/regex/v5/basic_regex.hpp:237:24: note: in instantiation of default argument for 'basic_string' required here
237 | result = std::basic_string(this->m_expression, this->m_expression_len);
| ^~~~~~~~~~~~~~~~~~~
Boost/1.83.0/include/boost/regex/v5/basic_regex.hpp:466:39: note: in instantiation of member function 'boost::re_detail_500::basic_regex_implementation<int, boost::icu_regex_traits>::str' requested here
466 | return m_pimpl.get() ? m_pimpl->str() : std::basic_string();
| ^
Boost/1.83.0/include/boost/regex/v5/basic_regex.hpp:536:14: note: in instantiation of member function 'boost::basic_regex<int, boost::icu_regex_traits>::str' requested here
536 | return str().compare(that.str());
| ^
Boost/1.83.0/include/boost/regex/v5/basic_regex.hpp:540:14: note: in instantiation of member function 'boost::basic_regex<int, boost::icu_regex_traits>::compare' requested here
540 | return compare(e) == 0;
| ^
main.cpp:9:38: note: in instantiation of member function 'boost::basic_regex<int, boost::icu_regex_traits>::operator==' requested here

The problem seems to be the character type UChar32 (which is actually int32_t) when Boost.Regex 1.83.0 is used in combination with the ICU library.

[pkl97]

@jzmaddock

I tested with a main branch version of the upcoming clang 19. The comparison problem is indeed fixed and the code works fine. Thanks for that!

However this simple program still fails to compile:

#include <iostream>
#include <boost/regex/icu.hpp>

int main()
{
    boost::make_u32regex(".*").str();

    return 0;
}

It cannot work because str() in the ICU case returns std::basic_string<UChar32> which is no longer supported by the compiler.
How should Boost.Regex users deal with this issue?

In our case we used str() to convert from UTF-32 to UTF-8 because this is our internal encoding. We implemented a workaround by performing an iteration over the regular expression (with begin()/end()) and converting each UTF-32 character individually to UTF-8. This way we avoid the need for an actual string type.

Wouldn't it be simpler (and maybe also cleaner) to just switch the ICU case to std::basic_string<char32_t> instead of using UChar32? Yes, there is a signedness issue, because UChar32 is not unsigned (although the name suggests otherwise). But this could be dealt with inside the library without affecting users. And char32_t is available since C++11, so it is not a new feature that nobody can use.