[llvm-bugs] [Bug 46371] New: operator>> function name lookup fails in fold expression std::cin >> ... >> args;

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=46371

            Bug ID: 46371
           Summary: operator>> function name lookup fails in fold
                    expression std::cin >> ... >> args;
           Product: clang
           Version: 10.0
          Hardware: PC
                OS: Windows NT
            Status: NEW
          Severity: normal
          Priority: P
         Component: C++17
          Assignee: unassignedclangbugs at nondot.org
          Reporter: zjsdut at 163.com
                CC: blitzrakete at gmail.com, erik.pilkington at gmail.com,
                    llvm-bugs at lists.llvm.org, richard-llvm at metafoo.co.uk

The following code

#include <iostream>
#include <vector>

namespace io {
template<typename T>
std::istream &operator>>(std::istream &in, std::vector<T> &vec) {
  for (auto &x : vec)
    in >> x;
  return in;
}

template<class... Args> void scan(Args &... args) {
  (std::cin >> ... >> args);
}
}// namespace io

int main() {
    std::vector<int> s(1), t(1);
    io::scan(s, t);
    std::cout << s[0] << ' ' << t[0] << '\n';
}

does not compile with clang 10.0.0:

<source>:13:16: error: call to function 'operator>>' that is neither visible in
the template definition nor found by argument-dependent lookup
  (std::cin >> ... >> args);
               ^
<source>:19:9: note: in instantiation of function template specialization
'io::scan<std::vector<int, std::allocator<int> >, std::vector<int,
std::allocator<int> > >' requested here
    io::scan(s, t);
        ^
<source>:6:15: note: 'operator>>' should be declared prior to the call site
std::istream &operator>>(std::istream &in, std::vector<T> &vec) {
              ^
1 error generated.

See https://godbolt.org/z/wTMzhU 

However, the same code compiles and runs correctly with gcc 9.3. Is this code
ill-formed?

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