Qt 5.9: MinGW Developer build does not compile

In file included from D:\dev\5-mingw-59\qt-mingw-59\qtbase\include\QtCore\5.9.0\QtCore/private/qjson_p.h:1:0,
                 from compiler/qv4compiler_p.h:55,
                 from compiler\qv4compiler.cpp:40:
D:/dev/5-mingw-59/qt-mingw-59/qtbase/src/corelib/json/qjson_p.h: In member function 'uint QV4::Compiler::JSUnitGenerator::registerIndexedGetterLookup()':
D:/dev/5-mingw-59/qt-mingw-59/qtbase/src/corelib/json/qjson_p.h:187:35: error: 'l.QJsonPrivate::qle_bitfield<0, 4>::val' is used uninitialized in this function [-Werror=uninitialized]
         uint i = qFromLittleEndian(val);
                                   ^
compiler\qv4compiler.cpp:110:26: note: 'l' was declared here
     CompiledData::Lookup l;
                          ^
In file included from D:\dev\5-mingw-59\qt-mingw-59\qtbase\include\QtCore\5.9.0\QtCore/private/qjson_p.h:1:0,
                 from compiler/qv4compiler_p.h:55,
                 from compiler\qv4compiler.cpp:40:
D:/dev/5-mingw-59/qt-mingw-59/qtbase/src/corelib/json/qjson_p.h: In member function 'uint QV4::Compiler::JSUnitGenerator::registerIndexedSetterLookup()':
D:/dev/5-mingw-59/qt-mingw-59/qtbase/src/corelib/json/qjson_p.h:187:35: error: 'l.QJsonPrivate::qle_bitfield<0, 4>::val' is used uninitialized in this function [-Werror=uninitialized]
         uint i = qFromLittleEndian(val);
                                   ^
compiler\qv4compiler.cpp:119:26: note: 'l' was declared here
     CompiledData::Lookup l;
                          ^
In file included from D:\dev\5-mingw-59\qt-mingw-59\qtbase\include\QtCore\5.9.0\QtCore/private/qjson_p.h:1:0,
                 from compiler/qv4compiler_p.h:55,
                 from compiler\qv4compiler.cpp:40:
D:/dev/5-mingw-59/qt-mingw-59/qtbase/src/corelib/json/qjson_p.h: In member function 'uint QV4::Compiler::JSUnitGenerator::registerGetterLookup(const QString&)':
D:/dev/5-mingw-59/qt-mingw-59/qtbase/src/corelib/json/qjson_p.h:187:35: error: 'l.QJsonPrivate::qle_bitfield<0, 4>::val' is used uninitialized in this function [-Werror=uninitialized]
         uint i = qFromLittleEndian(val);
                                   ^
compiler\qv4compiler.cpp:128:26: note: 'l' was declared here
     CompiledData::Lookup l;
                          ^
In file included from D:\dev\5-mingw-59\qt-mingw-59\qtbase\include\QtCore\5.9.0\QtCore/private/qjson_p.h:1:0,
                 from compiler/qv4compiler_p.h:55,
                 from compiler\qv4compiler.cpp:40:
D:/dev/5-mingw-59/qt-mingw-59/qtbase/src/corelib/json/qjson_p.h: In member function 'uint QV4::Compiler::JSUnitGenerator::registerSetterLookup(const QString&)':
D:/dev/5-mingw-59/qt-mingw-59/qtbase/src/corelib/json/qjson_p.h:187:35: error: 'l.QJsonPrivate::qle_bitfield<0, 4>::val' is used uninitialized in this function [-Werror=uninitialized]
         uint i = qFromLittleEndian(val);
                                   ^
compiler\qv4compiler.cpp:138:26: note: 'l' was declared here
     CompiledData::Lookup l;
                          ^
In file included from D:\dev\5-mingw-59\qt-mingw-59\qtbase\include\QtCore\5.9.0\QtCore/private/qjson_p.h:1:0,
                 from compiler/qv4compiler_p.h:55,
                 from compiler\qv4compiler.cpp:40:
D:/dev/5-mingw-59/qt-mingw-59/qtbase/src/corelib/json/qjson_p.h: In member function 'uint QV4::Compiler::JSUnitGenerator::registerGlobalGetterLookup(const QString&)':
D:/dev/5-mingw-59/qt-mingw-59/qtbase/src/corelib/json/qjson_p.h:187:35: error: 'l.QJsonPrivate::qle_bitfield<0, 4>::val' is used uninitialized in this function [-Werror=uninitialized]
         uint i = qFromLittleEndian(val);
                                   ^
compiler\qv4compiler.cpp:147:26: note: 'l' was declared here
     CompiledData::Lookup l;
                          ^
In file included from D:\dev\5-mingw-59\qt-mingw-59\qtbase\include\QtCore\5.9.0\QtCore/private/qjson_p.h:1:0,
                 from compiler/qv4compiler_p.h:55,
                 from compiler\qv4compiler.cpp:40:
D:/dev/5-mingw-59/qt-mingw-59/qtbase/src/corelib/json/qjson_p.h: In member function 'int QV4::Compiler::JSUnitGenerator::registerRegExp(QV4::IR::RegExp*)':
D:/dev/5-mingw-59/qt-mingw-59/qtbase/src/corelib/json/qjson_p.h:187:35: error: 're.QJsonPrivate::qle_bitfield<0, 4>::val' is used uninitialized in this function [-Werror=uninitialized]
         uint i = qFromLittleEndian(val);
                                   ^
compiler\qv4compiler.cpp:156:26: note: 're' was declared here
     CompiledData::RegExp re;
                          ^
cc1plus.exe: all warnings being treated as errors
cc1plus.exe: all warnings being treated as errors
Makefile.Debug:14643: recipe for target '.obj/debug/qv4compiler.o' failed
mingw32-make[4]: *** [.obj/debug/qv4compiler.o] Error 1
Makefile:34: recipe for target 'debug' failed
mingw32-make[3]: *** [debug] Error 2
Makefile:46: recipe for target 'sub-qml-make_first-ordered' failed
mingw32-make[2]: *** [sub-qml-make_first-ordered] Error 2
Makefile:41: recipe for target 'sub-src-make_first' failed
mingw32-make[1]: *** [sub-src-make_first] Error 2
Makefile:230: recipe for target 'module-qtdeclarative-make_first' failed
mingw32-make: *** [module-qtdeclarative-make_first] Error 2
mingw32-make failed 512: at d:\dev\5-mingw-59\qt-mingw-59\qtrepotools\bin\qt5_tool line 212.