https://gcc.gnu.org/bugzilla/show_bug.cgi?id=120305
--- Comment #16 from Jonathan Wakely <redi at gcc dot gnu.org> --- (In reply to aneris from comment #13) > (In reply to Jonathan Wakely from comment #12) > > tl;dr if you want to compile with strict -std=c++20 -pedantic settings, you > > need to strictly follow the rules of the standard. > > I understand, though, at the very least, couldn't this be made a warning for > -pedantic? I think that would be immensely useful. I don't see how. The library code is just C++ code, it doesn't know what your intention was when writing the code, or whether you wanted to use __int128 without warnings or not. And I don't think there's any preprocessor macro that the library can even check to see if you used -pedantic or not. > On another note, this code does compile with Clang. Apparently godbolt > defaults to libstdc++ for clang instead of libc++ and that caused all the > confusion. Yes, we've been asking them to make that more obvious for years: https://github.com/compiler-explorer/compiler-explorer/issues/3682 > Here: https://godbolt.org/z/j6fYMdjeo > > I did some digging and this is because std::is_integral_v<__int128_t> is > unconditionally true in libc++, unlike in libstdc++ where it's only true > when the GNU extensions are enabled. Yes, which is not conforming to the C++20 (and earlier) standards. It is allowed by C++23 though, and I plan to treat that as a DR for earlier standards. That's the topic of PR 96710. > So I was wondering, is it really out of the question to promote __int128_t > to an integer type by default like Clang? It'd make some code more > compatible. That's PR 96710. It was not allowed by the standard, but we fixed that: https://cplusplus.github.io/LWG/issue3828 (I reported that issue myself, specifically because I want to resolve this problem). (In reply to aneris from comment #14) > (In reply to aneris from comment #13) > > > I did some digging and this is because std::is_integral_v<__int128_t> is > > unconditionally true in libc++, unlike in libstdc++ where it's only true > > when the GNU extensions are enabled. > > Ok this is not it at all. > > looks like libc++ defaults to a long long unsigned as the difference type > for an iota_view with long unsigned. Right, for libc++ std::integral<__int128> is always true, but it also looks like they chose not to use the mechanisms introduced by https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/p1522r1.pdf for iota_view, so that range_difference_t<iota_view<uint64_t>> cannot represent the size of some views. https://godbolt.org/z/3M7sxh5qn
