I am still getting errors this time when compiling with MS15 or MINGW

Under upp 9701 it compiles OK with either compiler.
Under upp 9801 or 9861 it does not and I get
: error C2676: binary '<<=': 'Upp::String' does not define this operator or a conversion to a type acceptable to the predefined operator

ln<<= GGA2decAng(ln);

ln<<GGA2decAng(ln);