[libcxx-dev] Problem with __builtin_strlen among compilers

Yan Yichen via libcxx-dev libcxx-dev at lists.llvm.org
Mon Sep 9 06:41:58 PDT 2019

Hi, I'm using libc++ with gcc and have met a problem with code below:

#include <string_view>
constexpr bool test() {
    const char s1[] = "mave";
    std::string_view s2(s1);
    return true;
int main() {

Error message was:
$ g++ -nostdinc++ -I/home/yiyan/.local/include/c++/v1 -nodefaultlibs -lc++ -lc++abi -lm -lc -lgcc_s -lgcc -std=c++17 --compile -o /dev/null 1.cc
1.cc: In function 'int main()':
1.cc:8:23: error: non-constant condition for static assertion
    8 |     static_assert(test());
      |                   ~~~~^~
In file included from /home/yiyan/.local/include/c++/v1/string_view:175,
                 from 1.cc:1:
1.cc:8:23:   in 'constexpr' expansion of 'test()'
1.cc:4:27:   in 'constexpr' expansion of 's2.std::__1::basic_string_view<char>::basic_string_view(((const char*)(& s1)))'
/home/yiyan/.local/include/c++/v1/string_view:238:46:   in 'constexpr' expansion of 'std::__1::char_traits<char>::length(__s)'
/home/yiyan/.local/include/c++/v1/__string:217:69: error: '__builtin_strlen(((const char*)(& s1)))' is not a constant expression
  217 |     length(const char_type* __s)  _NOEXCEPT {return __builtin_strlen(__s);}
      |                                                     ~~~~~~~~~~~~~~~~^~~~~

Looks like the root cause is that __builtin_strlen is not constexpr in gcc.
Do you consider to fix this kind of issue?

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