Deprecated: char_traits<T> in LLVM 18

[nvanwyen]

As of LLVM 18, the binarystring.hxx has a deprecation warning ...

In file included from /usr/local/include/pqxx/pqxx:4:
In file included from /usr/local/include/pqxx/array.hxx:26:
In file included from /usr/local/include/pqxx/connection.hxx:38:
In file included from /usr/local/include/pqxx/params.hxx:21:
In file included from /usr/local/include/pqxx/internal/statement_parameters.hxx:22:
/usr/local/include/pqxx/binarystring.hxx:62:27: warning: 'char_traits<unsigned char>' is deprecated: char_traits<T> for T not equal to char, wchar_t, char8_t, char16_t or char32_t is non-standard and is provided for a temporary period. It will be removed in LLVM 18, so please migrate off of it. [-Wdeprecated-declarations]
  using value_type = std::char_traits<char_type>::char_type;
                          ^
/Library/Developer/CommandLineTools/SDKs/MacOSX14.0.sdk/usr/include/c++/v1/__string/char_traits.h:79:8: note: 'char_traits<unsigned char>' has been explicitly marked deprecated here
struct _LIBCPP_DEPRECATED_("char_traits<T> for T not equal to char, wchar_t, char8_t, char16_t or char32_t is non-standard and is provided for a temporary period. It will be removed in LLVM 18, so please migrate off of it.")
       ^
/Library/Developer/CommandLineTools/SDKs/MacOSX14.0.sdk/usr/include/c++/v1/__config:836:53: note: expanded from macro '_LIBCPP_DEPRECATED_'
#      define _LIBCPP_DEPRECATED_(m) __attribute__((deprecated(m)))

[tt4g]

Almost duplicate: #726

[jtv]

Thanks for noting that @tt4g. Is it not fundamentally the same issue — "generic std::char_traits is deprecated"?

[tt4g]

From the following link, it appears that LLVM has made a decision to remove the definition of std::char_traits<T> which is not defined in the C++ standard.

• https://discourse.llvm.org/t/deprecating-std-string-t-for-non-character-t/66779
• https://reviews.llvm.org/D138307

As #726 quotes, the std::char_traits<T> is not being deprecated.
Only the definition of std::char_traits<T>, which is not defined in the C++ standard, is being removed.

Relevant changelog entry from LLVM 17:

The base template for std::char_traits has been marked as deprecated and will be removed in LLVM 18. If you are using std::char_traits with types other than char, wchar_t, char8_t, char16_t, char32_t or a custom character type for which you specialized std::char_traits, your code will stop working when we remove the base template. The Standard does not mandate that a base template is provided, and such a base template is bound to be incorrect for some types, which could currently cause unexpected behavior while going undetected.

[jtv]

I think these are duplicates. I'll close this one. Workarounds are coming (one is: use std::vector<std::byte> instead) and in C++20, as of libpqxx 8.0, we'll have concepts so we can probably support "any type that's sufficiently similar."

[nvanwyen]

I made the temporary fix, using std::vector<std::byte> on my side and my testing is successful. Thank you, looking forward to pulling the latest for the permanent fix.