Default PrettyWriter constructor doesn't compile in gcc 9 and 10

[madeso]

Using the master branch: https://github.com/Tencent/rapidjson/tree/8f4c021fa2f1e001d2376095928fc0532adf2ae6
Both gcc 9.3.0 and 10.1.0 fails to compile, but not clang
C++17 "mode": target_compile_features(main PUBLIC cxx_std_17)

kDefaultLevelDepth used in the default PrettyWriter constructor is declared protected and gcc doesn't like this.

Code that will recreate this

#include <sstream>

#include "rapidjson/document.h"
#include "rapidjson/prettywriter.h"
#include "rapidjson/ostreamwrapper.h"

int main()
{
    std::ostringstream ss;
    rapidjson::OStreamWrapper osw(ss);
    rapidjson::PrettyWriter writer(osw);
    return 0;
}

Full output:

[1/2] Building CXX object CMakeFiles/main.dir/main.cc.o
FAILED: CMakeFiles/main.dir/main.cc.o 
/usr/bin/g++   -I../../ -g   -std=gnu++17 -MD -MT CMakeFiles/main.dir/main.cc.o -MF CMakeFiles/main.dir/main.cc.o.d -o CMakeFiles/main.dir/main.cc.o -c ../../main.cc
In file included from ../../main.cc:4:
../../rapidjson/prettywriter.h: In function ‘PrettyWriter(OutputStream&, StackAllocator*, std::size_t)-> rapidjson::PrettyWriter<OutputStream, SourceEncoding, TargetEncoding, StackAllocator, writeFlags> [with OutputStream = rapidjson::BasicOStreamWrapper<std::basic_ostream<char> >; SourceEncoding = rapidjson::UTF8<>; TargetEncoding = rapidjson::UTF8<>; StackAllocator = rapidjson::CrtAllocator; unsigned int writeFlags = 0; std::size_t = long unsigned int]’:
../../rapidjson/prettywriter.h:58:102: error: ‘const size_t rapidjson::Writer<rapidjson::BasicOStreamWrapper<std::basic_ostream<char> >, rapidjson::UTF8<>, rapidjson::UTF8<>, rapidjson::CrtAllocator, 0>::kDefaultLevelDepth’ is protected within this context
   58 |     explicit PrettyWriter(OutputStream& os, StackAllocator* allocator = 0, size_t levelDepth = Base::kDefaultLevelDepth) :
      |                                                                                                      ^~~~~~~~~~~~~~~~~~
In file included from ../../rapidjson/prettywriter.h:18,
                 from ../../main.cc:4:
../../rapidjson/writer.h:294:25: note: declared protected here
  294 |     static const size_t kDefaultLevelDepth = 32;
      |                         ^~~~~~~~~~~~~~~~~~
ninja: build stopped: subcommand failed.

[miloyip]

Can you test moving it into public, if success, make a PR?

[madeso]

Done. #1720

Sorry for missing the note in the pullrequest tab. I should just have created a pullrequest instead :)