Compile error with GCC 6.3.0

[asantoni]

I ran into a compile error on GCC 6.3.0 (Debian 9). The last line is the most relevant:

/stuff/vcpkg_installed/x64-linux/include/spdlog/common.h: In function 'constexpr spdlog::string_view_t spdlog::details::to_string_view(const memory_buf_t&)':
/stuff/vcpkg_installed/x64-linux/include/spdlog/common.h:351:42: error: call to non-constexpr function 'constexpr const T* fmt::v10::detail::buffer<T>::data() const [with T = char]'
     return spdlog::string_view_t{buf.data(), buf.size()};
                                  ~~~~~~~~^~
In file included from /stuff/vcpkg_installed/x64-linux/include/fmt/format.h:49:0,
                 from /stuff/vcpkg_installed/x64-linux/include/fmt/std.h:22,
                 from /stuff/src/logger.h:10,
                 from /stuff/src/redacted.h:15,
                 from /stuff/src/redacted.cpp:1:
/stuff/vcpkg_installed/x64-linux/include/fmt/core.h:845:22: note: 'constexpr const T* fmt::v10::detail::buffer<T>::data() const [with T = char]' is not usable as a constexpr function because:
   FMT_CONSTEXPR auto data() const noexcept -> const T* { return ptr_; }
                      ^~~~
/stuff/vcpkg_installed/x64-linux/include/fmt/core.h:845:22: error: enclosing class of constexpr non-static member function 'constexpr const T* fmt::v10::detail::buffer<T>::data() const [with T = char]' is not a literal type
/stuff/vcpkg_installed/x64-linux/include/fmt/core.h:798:29: note: 'fmt::v10::detail::buffer<char>' is not literal because:
 template <typename T> class buffer {
                             ^~~~~~
/stuff/vcpkg_installed/x64-linux/include/fmt/core.h:798:29: note:   'fmt::v10::detail::buffer<char>' is not an aggregate, does not have a trivial default constructor, and has no constexpr constructor that is not a copy or move constructor

This issue is similar to #3096

My patch that fixes this compile error is:

diff --git a/include/fmt/core.h b/include/fmt/core.h
index 1fe1388..f97c630 100755
--- a/include/fmt/core.h
+++ b/include/fmt/core.h
@@ -804,7 +804,7 @@ template <typename T> class buffer {
  protected:
   // Don't initialize ptr_ since it is not accessed to save a few cycles.
   FMT_MSC_WARNING(suppress : 26495)
-  buffer(size_t sz) noexcept : size_(sz), capacity_(sz) {}
+  FMT_CONSTEXPR buffer(size_t sz) noexcept : size_(sz), capacity_(sz) {}

   FMT_CONSTEXPR20 buffer(T* p = nullptr, size_t sz = 0, size_t cap = 0) noexcept
       : ptr_(p), size_(sz), capacity_(cap) {}

I'm not sure how to get a reproducer in godbolt. My code seems to be be blowing up on:

#include <fmt/std.h>

Full compiler version string is:

g++ --version
g++ (Debian 6.3.0-18+deb9u1) 6.3.0 20170516

Thanks!

[vitaut]

Could you submit a PR?

[asantoni]

No problem, pull request is open:
#3743

Thanks for your consideration!