https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78156
Bug ID: 78156
Summary: constexpr basic_string_view::basic_string_view(const
charT *) calls non-constexpr char_traits::length
Product: gcc
Version: 7.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: libstdc++
Assignee: unassigned at gcc dot gnu.org
Reporter: suokkos at gmail dot com
Target Milestone: ---
Simple test case fails to compile:
#if __has_include("string_view")
#include <string_view>
#elif __has_include("experimental/string_view")
#include <experimental/string_view>
namespace std {
using namespace experimental;
}
#endif
constexpr std::string_view foo("bar");
Result with /usr/lib/gcc-snapshot/bin/g++ -c -std=c++17 -Wall test.cc -o test.o
/usr/lib/gcc-snapshot/bin/g++ --version
g++ (Ubuntu 20161006-1ubuntu1) 7.0.0 20161006 (experimental) [trunk revision
240826]
In file included from test.cc:2:0:
test.cc:11:37: in constexpr expansion of
'std::basic_string_view<char>(((const char*)"bar"))'
/usr/lib/gcc-snapshot/include/c++/7.0.0/string_view:100:58: error: call to
non-constexpr function 'static std::size_t std::char_traits<char>::length(const
char_type*)'
: _M_len{__str == nullptr ? 0 : traits_type::length(__str)},
~~~~~~~~~~~~~~~~~~~^~~~~~~
Related code locations:
https://github.com/gcc-mirror/gcc/blob/master/libstdc%2B%2B-v3/include/experimental/string_view#L110
https://github.com/gcc-mirror/gcc/blob/master/libstdc%2B%2B-v3/include/bits/char_traits.h#L106
https://github.com/gcc-mirror/gcc/blob/master/libstdc%2B%2B-v3/include/bits/char_traits.h#L265
Possible fix that would require changes to the standard would changing
std::char_traits::length to be constexpr. I tested that change to my headers
and it works with small changes for this case. I don't know if adding constexpr
will cause any other problems except preventing std::char_traits::length from
calling strlen. But latest version is already using __builtin_strlen.
Does __builtin_strlen pass constexpr test?
Yes. Both gcc and clang managed to compile constexpr with builtin_strlen.
Alternative workarounds would be duplicating std::char_trait implementation
with constexpr alternatives that can be used to make string_view provide
constexpr constructor.
