The following program makes GDC crash:

class C3
{
  void f1()
  {
    class C0
    {
      void f2()
      {
        void f0()
        {
          class C0
          {
            void f0()
            {
              f1;
            }
          }
        }
      }
    }
  }
}

$ gdc test.d
test.d: In function ‘f0’:
test.d:15:15: internal compiler error: in get_decl_tree, at d/d-codegen.cc:501
               f1;
               ^
Please submit a full bug report,
with preprocessed source if appropriate.
See <file:///usr/share/doc/gcc-6/README.Bugs> for instructions.