Why is template argument deduction failing when specifying the std namespace

Question

I stumble upon a compilation error which I don't understand. In the following code snippet the option (1) does not compile. But the second option does. I thought both should give the same result. Can someone explain me what my mistake is here?

#include <string>
#include <cctype>
#include <algorithm>

int main() {
    std::string str{"bla"};
    std::count_if(str.begin(),  str.end(), std::isupper); // (1)
    std::count_if(str.begin(),  str.end(), ::isupper);    // (2)
}

The error message:

../src/main.cpp: In function ‘int main()’:
../src/main.cpp:7:18: error: no matching function for call to ‘count_if(std::__cxx11::basic_string<char>::iterator, std::__cxx11::basic_string<char>::iterator, <unresolved overloaded function type>)’
    7 |     std::count_if(str.begin(),  str.end(), std::isupper); // (1)
      |     ~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from /usr/include/c++/11.1.0/algorithm:62,
                 from ../src/main.cpp:3:
/usr/include/c++/11.1.0/bits/stl_algo.h:4083:5: note: candidate: ‘template<class _IIter, class _Predicate> typename std::iterator_traits< <template-parameter-1-1> >::difference_type std::count_if(_IIter, _IIter, _Predicate)’
 4083 |     count_if(_InputIterator __first, _InputIterator __last, _Predicate __pred)
      |     ^~~~~~~~
/usr/include/c++/11.1.0/bits/stl_algo.h:4083:5: note:   template argument deduction/substitution failed:
../src/main.cpp:7:18: note:   couldn’t deduce template parameter ‘_Predicate’
    7 |     std::count_if(str.begin(),  str.end(), std::isupper); // (1)
      |     ~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from /usr/include/c++/11.1.0/algorithm:74,
                 from ../src/main.cpp:3:
/usr/include/c++/11.1.0/pstl/glue_algorithm_defs.h:106:1: note: candidate: ‘template<class _ExecutionPolicy, class _ForwardIterator, class _Predicate> __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, typename std::iterator_traits<_II>::difference_type> std::count_if(_ExecutionPolicy&&, _ForwardIterator, _ForwardIterator, _Predicate)’
  106 | count_if(_ExecutionPolicy&& __exec, _ForwardIterator __first, _ForwardIterator __last, _Predicate __pred);
      | ^~~~~~~~
/usr/include/c++/11.1.0/pstl/glue_algorithm_defs.h:106:1: note:   template argument deduction/substitution failed:
../src/main.cpp:7:18: note:   candidate expects 4 arguments, 3 provided
    7 |     std::count_if(str.begin(),  str.end(), std::isupper); // (1)
      |     ~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~