GCC 13.1 compile error when using CXXFLAGS=-std=c++20

[Helmut Zeisel via Gcc]

I compiled GCC 13.1.0 with GCC 12 and had the environment variable CXXFLAGS set 
to -std=c++20
This gives the error (both linux and cygin)

gcc-13.1.0/libstdc++-v3/src/c++98/bitmap_allocator.cc:51:23: error: ISO C++17 
does not allow dynamic exception specifications
   51 |   _M_get(size_t __sz) throw(std::bad_alloc)

After

   unset CXXFLAGS

compilation works

Helmut