Temporary std::string in template argument is not considert a constant expression in C++20 mode #62462
Description
This is a bug in clang++. I tested with libc++ and libstdc++ which both give similar error messages.
#include <algorithm>
#include <string>
template <std::size_t N>
struct ct_to_rt_string {
consteval ct_to_rt_string(std::string const str) {
std::copy_n(str.c_str(), N + 1, buffer);
}
char buffer[N + 1];
};
constexpr std::string make_ct_std_string() {
return "compile time text";
}
#include <iostream>
int main() {
std::cout << ct_to_rt_string<make_ct_std_string().size()>(make_ct_std_string()).buffer;
}GCC 12.2 compiles this code and prints compile time text as expected.
clang++ -std=c++20 -stdlib=libc++ fails to compile with:
<source>:20:34: error: non-type template argument is not a constant expression
std::cout << ct_to_rt_string<make_ct_std_string().size()>(make_ct_std_string()).buffer;
^~~~~~~~~~~~~~~~~~~~~~~~~~~
/opt/compiler-explorer/clang-16.0.0/bin/../include/c++/v1/__memory/allocator.h:113:38: note: allocation performed here was not deallocated
return static_cast<_Tp*>(::operator new(__n * sizeof(_Tp)));
^
1 error generated.
ASM generation compiler returned: 1
<source>:20:34: error: non-type template argument is not a constant expression
std::cout << ct_to_rt_string<make_ct_std_string().size()>(make_ct_std_string()).buffer;
^~~~~~~~~~~~~~~~~~~~~~~~~~~
/opt/compiler-explorer/clang-16.0.0/bin/../include/c++/v1/__memory/allocator.h:113:38: note: allocation performed here was not deallocated
return static_cast<_Tp*>(::operator new(__n * sizeof(_Tp)));
^
1 error generated.Clang++ does accept the code if you change the call to: (Found by Paul Sanders at Stackoverflow)
int main() {
constexpr size_t N = make_ct_std_string().size();
std::cout << ct_to_rt_string<N>(make_ct_std_string()).buffer;
}I tested with LLVM 16.0.0: