<html><head><meta http-equiv="Content-Type" content="text/html; charset=utf-8"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; line-break: after-white-space;" class="">I think the most productive way of resolving this issue is to file a bug against GCC asking them to make __builtin_strlen constexpr.<div class=""><br class=""></div><div class="">Alternatively, libc++ could also use `if (std::is_constant_evaluated())` to pick between __builtin_strlen and a naive version, but that won't fix your problem when compiling in C++17 mode because is_constant_evaluated() is (will be) C++20.</div><div class=""><br class=""></div><div class="">Louis</div><div class=""><div><br class=""><blockquote type="cite" class=""><div class="">On Sep 9, 2019, at 09:41, Yan Yichen via libcxx-dev <<a href="mailto:libcxx-dev@lists.llvm.org" class="">libcxx-dev@lists.llvm.org</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><div class="WordSection1" style="page: WordSection1; caret-color: rgb(0, 0, 0); font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration: none;"><div style="margin: 0in 0in 0.0001pt; font-size: 11pt; font-family: Calibri, sans-serif;" class=""><span lang="EN-US" class="">Hi, I’m using libc++ with gcc and have met a problem with code below:<o:p class=""></o:p></span></div><div style="margin: 0in 0in 0.0001pt; font-size: 11pt; font-family: Calibri, sans-serif;" class=""><span lang="EN-US" class=""><o:p class=""> </o:p></span></div><div style="margin: 0in 0in 0.0001pt; font-size: 11pt; font-family: Calibri, sans-serif;" class=""><span lang="EN-US" class=""><o:p class=""> </o:p></span></div><div style="margin: 0in 0in 0.0001pt; font-size: 11pt; font-family: Calibri, sans-serif;" class=""><span lang="EN-US" style="font-family: Consolas;" class="">#include <string_view><o:p class=""></o:p></span></div><div style="margin: 0in 0in 0.0001pt; font-size: 11pt; font-family: Calibri, sans-serif;" class=""><span lang="EN-US" style="font-family: Consolas;" class="">constexpr bool test() {<o:p class=""></o:p></span></div><div style="margin: 0in 0in 0.0001pt; font-size: 11pt; font-family: Calibri, sans-serif;" class=""><span lang="EN-US" style="font-family: Consolas;" class=""> const char s1[] = "mave";<o:p class=""></o:p></span></div><div style="margin: 0in 0in 0.0001pt; font-size: 11pt; font-family: Calibri, sans-serif;" class=""><span lang="EN-US" style="font-family: Consolas;" class=""> std::string_view s2(s1);<o:p class=""></o:p></span></div><div style="margin: 0in 0in 0.0001pt; font-size: 11pt; font-family: Calibri, sans-serif;" class=""><span lang="EN-US" style="font-family: Consolas;" class=""> return true;<o:p class=""></o:p></span></div><div style="margin: 0in 0in 0.0001pt; font-size: 11pt; font-family: Calibri, sans-serif;" class=""><span lang="EN-US" style="font-family: Consolas;" class="">}<o:p class=""></o:p></span></div><div style="margin: 0in 0in 0.0001pt; font-size: 11pt; font-family: Calibri, sans-serif;" class=""><span lang="EN-US" style="font-family: Consolas;" class="">int main() {<o:p class=""></o:p></span></div><div style="margin: 0in 0in 0.0001pt; font-size: 11pt; font-family: Calibri, sans-serif;" class=""><span lang="EN-US" style="font-family: Consolas;" class=""> static_assert(test());<o:p class=""></o:p></span></div><div style="margin: 0in 0in 0.0001pt; font-size: 11pt; font-family: Calibri, sans-serif;" class=""><span lang="EN-US" style="font-family: Consolas;" class="">}<o:p class=""></o:p></span></div><div style="margin: 0in 0in 0.0001pt; font-size: 11pt; font-family: Calibri, sans-serif;" class=""><span lang="EN-US" class=""><o:p class=""> </o:p></span></div><div style="margin: 0in 0in 0.0001pt; font-size: 11pt; font-family: Calibri, sans-serif;" class=""><span lang="EN-US" class=""><o:p class=""> </o:p></span></div><div style="margin: 0in 0in 0.0001pt; font-size: 11pt; font-family: Calibri, sans-serif;" class=""><span lang="EN-US" class="">Error message was:<o:p class=""></o:p></span></div><div style="margin: 0in 0in 0.0001pt; font-size: 11pt; font-family: Calibri, sans-serif;" class=""><span lang="EN-US" style="font-family: Consolas;" class="">$ 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<span class="Apple-converted-space"> </span><a href="http://1.cc/" style="color: rgb(149, 79, 114); text-decoration: underline;" class="">1.cc</a><o:p class=""></o:p></span></div><div style="margin: 0in 0in 0.0001pt; font-size: 11pt; font-family: Calibri, sans-serif;" class=""><span lang="EN-US" style="font-family: Consolas;" class=""><a href="http://1.cc/" style="color: rgb(149, 79, 114); text-decoration: underline;" class="">1.cc</a>: In function ‘int main()’:<o:p class=""></o:p></span></div><div style="margin: 0in 0in 0.0001pt; font-size: 11pt; font-family: Calibri, sans-serif;" class=""><span lang="EN-US" style="font-family: Consolas;" class=""><a href="http://1.cc/" style="color: rgb(149, 79, 114); text-decoration: underline;" class="">1.cc</a>:8:23: error: non-constant condition for static assertion<o:p class=""></o:p></span></div><div style="margin: 0in 0in 0.0001pt; font-size: 11pt; font-family: Calibri, sans-serif;" class=""><span lang="EN-US" style="font-family: Consolas;" class=""> 8 | static_assert(test());<o:p class=""></o:p></span></div><div style="margin: 0in 0in 0.0001pt; font-size: 11pt; font-family: Calibri, sans-serif;" class=""><span lang="EN-US" style="font-family: Consolas;" class=""> | ~~~~^~<o:p class=""></o:p></span></div><div style="margin: 0in 0in 0.0001pt; font-size: 11pt; font-family: Calibri, sans-serif;" class=""><span lang="EN-US" style="font-family: Consolas;" class="">In file included from /home/yiyan/.local/include/c++/v1/string_view:175,<o:p class=""></o:p></span></div><div style="margin: 0in 0in 0.0001pt; font-size: 11pt; font-family: Calibri, sans-serif;" class=""><span lang="EN-US" style="font-family: Consolas;" class=""> from<span class="Apple-converted-space"> </span><a href="http://1.cc/" style="color: rgb(149, 79, 114); text-decoration: underline;" class="">1.cc</a>:1:<o:p class=""></o:p></span></div><div style="margin: 0in 0in 0.0001pt; font-size: 11pt; font-family: Calibri, sans-serif;" class=""><span lang="EN-US" style="font-family: Consolas;" class=""><a href="http://1.cc/" style="color: rgb(149, 79, 114); text-decoration: underline;" class="">1.cc</a>:8:23: in ‘constexpr’ expansion of ‘test()’<o:p class=""></o:p></span></div><div style="margin: 0in 0in 0.0001pt; font-size: 11pt; font-family: Calibri, sans-serif;" class=""><span lang="EN-US" style="font-family: Consolas;" class=""><a href="http://1.cc/" style="color: rgb(149, 79, 114); text-decoration: underline;" class="">1.cc</a>:4:27: in ‘constexpr’ expansion of ‘s2.std::__1::basic_string_view<char>::basic_string_view(((const char*)(& s1)))’<o:p class=""></o:p></span></div><div style="margin: 0in 0in 0.0001pt; font-size: 11pt; font-family: Calibri, sans-serif;" class=""><span lang="EN-US" style="font-family: Consolas;" class="">/home/yiyan/.local/include/c++/v1/string_view:238:46: in ‘constexpr’ expansion of ‘std::__1::char_traits<char>::length(__s)’<o:p class=""></o:p></span></div><div style="margin: 0in 0in 0.0001pt; font-size: 11pt; font-family: Calibri, sans-serif;" class=""><span lang="EN-US" style="font-family: Consolas;" class="">/home/yiyan/.local/include/c++/v1/__string:217:69: error: ‘__builtin_strlen(((const char*)(& s1)))’ is not a constant expression<o:p class=""></o:p></span></div><div style="margin: 0in 0in 0.0001pt; font-size: 11pt; font-family: Calibri, sans-serif;" class=""><span lang="EN-US" style="font-family: Consolas;" class=""> 217 | length(const char_type* __s) _NOEXCEPT {return __builtin_strlen(__s);}<o:p class=""></o:p></span></div><div style="margin: 0in 0in 0.0001pt; font-size: 11pt; font-family: Calibri, sans-serif;" class=""><span lang="EN-US" style="font-family: Consolas;" class=""> | ~~~~~~~~~~~~~~~~^~~~~<o:p class=""></o:p></span></div><div style="margin: 0in 0in 0.0001pt; font-size: 11pt; font-family: Calibri, sans-serif;" class=""><span lang="EN-US" class=""><o:p class=""> </o:p></span></div><div style="margin: 0in 0in 0.0001pt; font-size: 11pt; font-family: Calibri, sans-serif;" class=""><span lang="EN-US" class=""><o:p class=""> </o:p></span></div><div style="margin: 0in 0in 0.0001pt; font-size: 11pt; font-family: Calibri, sans-serif;" class=""><span lang="EN-US" class="">Looks like the root cause is that<span class="Apple-converted-space"> </span></span><span lang="EN-US" style="font-family: Consolas;" class="">__builtin_strlen</span><span lang="EN-US" class=""><span class="Apple-converted-space"> </span>is not constexpr in gcc.<o:p class=""></o:p></span></div><div style="margin: 0in 0in 0.0001pt; font-size: 11pt; font-family: Calibri, sans-serif;" class=""><span lang="EN-US" class="">Do you consider to fix this kind of issue?<o:p class=""></o:p></span></div><div style="margin: 0in 0in 0.0001pt; font-size: 11pt; font-family: Calibri, sans-serif;" class=""><span lang="EN-US" class=""><o:p class=""> </o:p></span></div><div style="margin: 0in 0in 0.0001pt; font-size: 11pt; font-family: Calibri, sans-serif;" class=""><span lang="EN-US" class=""><o:p class=""> </o:p></span></div><div style="margin: 0in 0in 0.0001pt; font-size: 11pt; font-family: Calibri, sans-serif;" class=""><span lang="EN-US" class="">Best,<o:p class=""></o:p></span></div><div style="margin: 0in 0in 0.0001pt; font-size: 11pt; font-family: Calibri, sans-serif;" class=""><span lang="EN-US" class="">Yichen<o:p class=""></o:p></span></div></div><span style="caret-color: rgb(0, 0, 0); font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration: none; float: none; display: inline !important;" class="">_______________________________________________</span><br style="caret-color: rgb(0, 0, 0); font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration: none;" class=""><span style="caret-color: rgb(0, 0, 0); font-family: Helvetica; font-size: 12px; font-style: normal; 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