https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112642
Bug ID: 112642 Summary: ranges::fold_left tries to access inactive union member of string in constant expression Product: gcc Version: 13.2.1 Status: UNCONFIRMED Severity: normal Priority: P3 Component: libstdc++ Assignee: unassigned at gcc dot gnu.org Reporter: miro.palmu at helsinki dot fi Target Milestone: --- Following example will fail to compile. Error message is included below but most important part is here: error: accessing 'std::__cxx11::basic_string<char>::<unnamed union>::_M_allocated_capacity' member instead of initialized 'std::__cxx11::basic_string<char>::<unnamed union>::_M_local_buf' member in constant expression Example: #include <string> #include <functional> #include <algorithm> #include <vector> using namespace std; using namespace std::literals; constexpr auto foo() { const auto vec = vector{ "a"s, "b"s, "c"s }; const auto concat = ranges::fold_left(vec, ""s, plus{}); return concat.size(); } int main() { constexpr auto _ = foo(); } Error message: Using built-in specs. COLLECT_GCC=g++ COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-pc-linux-gnu/13.2.1/lto-wrapper Target: x86_64-pc-linux-gnu Configured with: /build/gcc/src/gcc/configure --enable-languages=ada,c,c++,d,fortran,go,lto,objc,obj-c++ --enable-bootstrap --prefix=/usr --libdir=/usr/lib --libexecdir=/usr/lib --mandir=/usr/share/man --infodir=/usr/share/info --with-bugurl=https://bugs.archlinux.org/ --with-build-config=bootstrap-lto --with-linker-hash-style=gnu --with-system-zlib --enable-__cxa_atexit --enable-cet=auto --enable-checking=release --enable-clocale=gnu --enable-default-pie --enable-default-ssp --enable-gnu-indirect-function --enable-gnu-unique-object --enable-libstdcxx-backtrace --enable-link-serialization=1 --enable-linker-build-id --enable-lto --enable-multilib --enable-plugin --enable-shared --enable-threads=posix --disable-libssp --disable-libstdcxx-pch --disable-werror Thread model: posix Supported LTO compression algorithms: zlib zstd gcc version 13.2.1 20230801 (GCC) COLLECT_GCC_OPTIONS='-v' '-save-temps' '-std=c++23' '-shared-libgcc' '-mtune=generic' '-march=x86-64' '-dumpdir' 'a-' /usr/lib/gcc/x86_64-pc-linux-gnu/13.2.1/cc1plus -E -quiet -v -D_GNU_SOURCE prog.cpp -mtune=generic -march=x86-64 -std=c++23 -fpch-preprocess -o a-prog.ii ignoring nonexistent directory "/usr/lib/gcc/x86_64-pc-linux-gnu/13.2.1/../../../../x86_64-pc-linux-gnu/include" #include "..." search starts here: #include <...> search starts here: /usr/lib/gcc/x86_64-pc-linux-gnu/13.2.1/../../../../include/c++/13.2.1 /usr/lib/gcc/x86_64-pc-linux-gnu/13.2.1/../../../../include/c++/13.2.1/x86_64-pc-linux-gnu /usr/lib/gcc/x86_64-pc-linux-gnu/13.2.1/../../../../include/c++/13.2.1/backward /usr/lib/gcc/x86_64-pc-linux-gnu/13.2.1/include /usr/local/include /usr/lib/gcc/x86_64-pc-linux-gnu/13.2.1/include-fixed /usr/include End of search list. COLLECT_GCC_OPTIONS='-v' '-save-temps' '-std=c++23' '-shared-libgcc' '-mtune=generic' '-march=x86-64' '-dumpdir' 'a-' /usr/lib/gcc/x86_64-pc-linux-gnu/13.2.1/cc1plus -fpreprocessed a-prog.ii -quiet -dumpdir a- -dumpbase prog.cpp -dumpbase-ext .cpp -mtune=generic -march=x86-64 -std=c++23 -version -o a-prog.s GNU C++23 (GCC) version 13.2.1 20230801 (x86_64-pc-linux-gnu) compiled by GNU C version 13.2.1 20230801, GMP version 6.3.0, MPFR version 4.2.0-p12, MPC version 1.3.1, isl version isl-0.26-GMP warning: MPFR header version 4.2.0-p12 differs from library version 4.2.1. GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072 Compiler executable checksum: 5a490a353c29b926850bca65a518c219 prog.cpp: In function ‘int main()’: prog.cpp:16:27: in ‘constexpr’ expansion of ‘foo()’ prog.cpp:16:28: in ‘constexpr’ expansion of ‘std::ranges::__fold_left_fn::operator()(_Range&&, _Tp, _Fp) const [with _Range = const std::vector<std::__cxx11::basic_string<char>, std::allocator<std::__cxx11::basic_string<char> > >&; _Tp = std::__cxx11::basic_string<char>; _Fp = std::plus<void>](vec, std::literals::string_literals::operator""s(const char*, std::size_t)(0), (std::plus<void>(), std::plus<void>()))’ prog.cpp:16:28: in ‘constexpr’ expansion of ‘std::__cxx11::basic_string<char>((* & std::move<__cxx11::basic_string<char>&>(__init)))’ prog.cpp:16:28: error: accessing ‘std::__cxx11::basic_string<char>::<unnamed union>::_M_allocated_capacity’ member instead of initialized ‘std::__cxx11::basic_string<char>::<unnamed union>::_M_local_buf’ member in constant expression 16 | constexpr auto _ = foo(); | ^