Some compilers will fail to build with "undeclared identifier 'digit_to_number'" error

[tt4g]

There are some compilers that fail to compile when building version 7.7.0.

This problem was encountered in a CI build failure of the package manager conan (conan-io/conan-center-index#8761 (comment)).

The following is the error log when building with Clang.

CI Failure log

[...]
[ 14%] Building CXX object source_subfolder/src/CMakeFiles/pqxx.dir/connection.cxx.o
[ 18%] Building CXX object source_subfolder/src/CMakeFiles/pqxx.dir/cursor.cxx.o
[ 22%] Building CXX object source_subfolder/src/CMakeFiles/pqxx.dir/encodings.cxx.o
[ 25%] Building CXX object source_subfolder/src/CMakeFiles/pqxx.dir/errorhandler.cxx.o
[ 29%] Building CXX object source_subfolder/src/CMakeFiles/pqxx.dir/except.cxx.o
[ 33%] Building CXX object source_subfolder/src/CMakeFiles/pqxx.dir/field.cxx.o
[ 37%] Building CXX object source_subfolder/src/CMakeFiles/pqxx.dir/largeobject.cxx.o
[ 40%] Building CXX object source_subfolder/src/CMakeFiles/pqxx.dir/notification.cxx.o
[ 44%] Building CXX object source_subfolder/src/CMakeFiles/pqxx.dir/params.cxx.o
[ 48%] Building CXX object source_subfolder/src/CMakeFiles/pqxx.dir/pipeline.cxx.o
[ 51%] Building CXX object source_subfolder/src/CMakeFiles/pqxx.dir/result.cxx.o
[ 55%] Building CXX object source_subfolder/src/CMakeFiles/pqxx.dir/robusttransaction.cxx.o
[ 59%] Building CXX object source_subfolder/src/CMakeFiles/pqxx.dir/row.cxx.o
[ 62%] Building CXX object source_subfolder/src/CMakeFiles/pqxx.dir/sql_cursor.cxx.o
[ 66%] Building CXX object source_subfolder/src/CMakeFiles/pqxx.dir/strconv.cxx.o
source_subfolder/src/CMakeFiles/pqxx.dir/build.make:302: recipe for target 'source_subfolder/src/CMakeFiles/pqxx.dir/strconv.cxx.o' failed
CMakeFiles/Makefile2:151: recipe for target 'source_subfolder/src/CMakeFiles/pqxx.dir/all' failed
Makefile:170: recipe for target 'all' failed
libpqxx/7.7.0: 
CMake Warning:
  Manually-specified variables were not used by the project:

    CMAKE_EXPORT_NO_PACKAGE_REGISTRY


/home/conan/w/BuildSingleReference/.conan/data/libpqxx/7.7.0/_/_/build/e52f527ce44f8ad45ccc34f94acd11eaa754d477/source_subfolder/src/strconv.cxx:441:46: error: use of undeclared identifier 'digit_to_number'; did you mean 'pqxx::internal::digit_to_number'?
      result = absorb_digit_positive(result, digit_to_number(data[i]));
                                             ^~~~~~~~~~~~~~~
                                             pqxx::internal::digit_to_number
/home/conan/w/BuildSingleReference/.conan/data/libpqxx/7.7.0/_/_/build/e52f527ce44f8ad45ccc34f94acd11eaa754d477/source_subfolder/include/pqxx/internal/conversions.hxx:34:15: note: 'pqxx::internal::digit_to_number' declared here
constexpr int digit_to_number(char c) noexcept
              ^
/home/conan/w/BuildSingleReference/.conan/data/libpqxx/7.7.0/_/_/build/e52f527ce44f8ad45ccc34f94acd11eaa754d477/source_subfolder/src/strconv.cxx:455:46: error: use of undeclared identifier 'digit_to_number'; did you mean 'pqxx::internal::digit_to_number'?
      result = absorb_digit_negative(result, digit_to_number(data[i]));
                                             ^~~~~~~~~~~~~~~
                                             pqxx::internal::digit_to_number
/home/conan/w/BuildSingleReference/.conan/data/libpqxx/7.7.0/_/_/build/e52f527ce44f8ad45ccc34f94acd11eaa754d477/source_subfolder/include/pqxx/internal/conversions.hxx:34:15: note: 'pqxx::internal::digit_to_number' declared here
constexpr int digit_to_number(char c) noexcept
              ^
2 errors generated.
make[2]: *** [source_subfolder/src/CMakeFiles/pqxx.dir/strconv.cxx.o] Error 1
make[2]: *** Waiting for unfinished jobs....
make[1]: *** [source_subfolder/src/CMakeFiles/pqxx.dir/all] Error 2
make: *** [all] Error 2
libpqxx/7.7.0: WARN: Build folder is dirty, removing it: /home/conan/w/BuildSingleReference/.conan/data/libpqxx/7.7.0/_/_/build/e52f527ce44f8ad45ccc34f94acd11eaa754d477
libpqxx/7.7.0: ERROR: Package 'e52f527ce44f8ad45ccc34f94acd11eaa754d477' build failed
libpqxx/7.7.0: WARN: Build folder /home/conan/w/BuildSingleReference/.conan/data/libpqxx/7.7.0/_/_/build/e52f527ce44f8ad45ccc34f94acd11eaa754d477
ERROR: libpqxx/7.7.0: Error in build() method, line 101
	cmake.build()
	ConanException: Error 2 while ex

[tt4g]

It seems that it is after 6dec39a that the compiler can no longer find digit_to_number.

Before this commit, digit_to_number was defined in anonymous namespace.
After this commit, digit_to_number is defined in the pqxx::internal namespace, so the compiler cannot find digit_to_number.

[tt4g]

I used a few Docker clang 11 images to fix this problem.
Oddly enough, I noticed that the problem was not reproduced in some clang images.

The difference may be due to different patch versions of clang, but we could not track down the cause of the failure to reproduce the problem.

[jtv]

Weird that my clang versions don't complain about this. Oh well, fixed now!