https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107862
Bug ID: 107862 Summary: Returning an std::vector from a lambda fails to be constexpr, while a custom class with allocated storage works Product: gcc Version: 12.2.0 URL: https://godbolt.org/z/dve3Yx8ax Status: UNCONFIRMED Severity: normal Priority: P3 Component: c++ Assignee: unassigned at gcc dot gnu.org Reporter: milasudril at gmail dot com Target Milestone: --- Host: x86-64_linux_gnu Target: x86-64_linux_gnu Minimal working example: #include <array> #include <algorithm> #include <vector> struct Test { using value_type = int; constexpr Test(){ data = new value_type(0); } constexpr ~Test(){ delete data; } constexpr unsigned size() const { return 1; } constexpr auto begin() const { return data; } constexpr auto end() const { return data + 1; } private: value_type* data; }; consteval auto dynamic_data_to_array(auto generator) { auto test = generator(); using VT = typename decltype(test)::value_type; std::array<VT, test.size()> data; std::copy(test.begin(), test.end(), data.begin()); return data; } constexpr Test get_test() { return Test(); } int main() { // Does not work constexpr auto data0 = dynamic_data_to_array([] { return std::vector<int>{0};}); constexpr auto data1 = dynamic_data_to_array([] { return Test{};}); return data1[0]; } It works to return a Test object here, but not an std::vector. Compiler output: <source>:22:29: in 'constexpr' expansion of 'test.std::vector<int>::size()' <source>:22:33: error: the value of 'test' is not usable in a constant expression 22 | std::array<VT, test.size()> data; | ^~~~ <source>:19:10: note: 'test' was not declared 'constexpr' 19 | auto test = generator(); | ^~~~ <source>:22:29: note: in template argument for type 'long unsigned int' 22 | std::array<VT, test.size()> data; | ~~~~~~~~~^~ <source>:22:29: in 'constexpr' expansion of 'test.std::vector<int>::size()' <source>:22:33: error: the value of 'test' is not usable in a constant expression 22 | std::array<VT, test.size()> data; | ^~~~ <source>:19:10: note: 'test' was not declared 'constexpr' 19 | auto test = generator(); | ^~~~ <source>:22:29: note: in template argument for type 'long unsigned int' 22 | std::array<VT, test.size()> data; I am not sure if this is by the standard, or if this is related to some missing implementation details in the standard library. Note: clang doesn't like it either.