On Fri, 3 Dec 2021 at 22:38, Jonathan Wakely wrote: > > On Mon, 22 Nov 2021 at 16:31, Stephan Bergmann via Libstdc++ > <libstd...@gcc.gnu.org> wrote: > > > > When using recent libstc++ trunk with Clang in C++20 mode, > > std::u16string literals as in > > > > > #include <string> > > > int main() { > > > using namespace std::literals; > > > u""s; > > > } > > > > started to cause linker failures due to undefined > > > > > _ZNSt7__cxx1112basic_stringIDsSt11char_traitsIDsESaIDsEE12_M_constructIPKDsEEvT_S8_St20forward_iterator_tag > > > > After some head scratching, I found the more insightful > > > > > $ cat test.cc > > > #include <string> > > > constexpr std::string s("", 0); > > > > > $ clang++ -std=c++20 -fsyntax-only test.cc > > > test.cc:2:23: error: constexpr variable 's' must be initialized by a > > > constant expression > > > constexpr std::string s("", 0); > > > ^~~~~~~~ > > > ~/gcc/trunk/inst/lib/gcc/x86_64-pc-linux-gnu/12.0.0/../../../../include/c++/12.0.0/bits/basic_string.h:620:2: > > > note: undefined function '_M_construct<const char *>' cannot be used in > > > a constant expression > > > _M_construct(__s, __s + __n, std::forward_iterator_tag()); > > > ^ > > > test.cc:2:23: note: in call to 'basic_string(&""[0], 0, > > > std::allocator<char>())' > > > constexpr std::string s("", 0); > > > ^ > > > ~/gcc/trunk/inst/lib/gcc/x86_64-pc-linux-gnu/12.0.0/../../../../include/c++/12.0.0/bits/basic_string.h:331:9: > > > note: declared here > > > _M_construct(_FwdIterator __beg, _FwdIterator __end, > > > ^ > > > 1 error generated. > > > > and after some more head scratching found Clang to complain about the > > reduced > > > > > template<typename> struct S { > > > constexpr void f(); > > > constexpr S() { f(); }; > > > }; > > > S<void> s1; > > > template<typename T> constexpr void S<T>::f() {} > > > constexpr S<void> s2; > > > > (about which GCC does not complain). Not entirely sure who is right, > > but what would help Clang is to move the definitions of the literal > > operators in basic_string.h (which implicitly instantiate the > > corresponding std::basic_string<_Tp> constructor) past the definition of > > _M_construct (which is called from the constructor) in basic_string.tcc; > > something like > > The .tcc files are something of an anachronism, as I think they were > supposed to have the non-inline function definitions which might be > subject to 'export' for separate compilation. Except that feature was > removed from C++11, and so now it's just a fairly pointless separation > between inline and non-inline functions ... except where we're muddied > the waters by changing some to 'inline' without moving them to the > other file (because why bother). > > That said, all the one- or two-line inline functions like the literal > operators and to_string are all in basic_string.h and having to move > some arbitrary subset of them into the other file, after the > non-inline definitions, is a bit annoying. > > I think this is https://bugs.llvm.org/show_bug.cgi?id=24128
Which is now https://github.com/llvm/llvm-project/issues/24502