algorithm.cpp failed with clang-cl on Windows for "declaration does not declare anything"

[yuxianch]

With the latest version of clang-cl, algorithm.cpp get failed with clang-cl on Windows for "declaration does not declare anything". Related code: https://github.com/boostorg/range/blob/72e2a4457a13eba0b8cddd2e68f058f74bd3d890/test/algorithm.cpp#L32-L35 A small reproducer(test.cpp):

namespace boost {
  template <class T>
  class counting_iterator
  {};
}
template ::boost::counting_iterator<int>;

Error message:

ksh-3.2$ clang-cl -c test.cpp
test.cpp(6,10): error: declaration does not declare anything
template ::boost::counting_iterator<int>;
         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1 error generated.

The code is considered as not accepted(https://github.com/llvm/llvm-project/commit/1cbdf932b41eb58ed94fbc240e93b63653d95bd3).