[llvm-bugs] [Bug 46577] New: Operator overload resolution fails in fold expressions

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=46577

            Bug ID: 46577
           Summary: Operator overload resolution fails in fold expressions
           Product: clang
           Version: 10.0
          Hardware: PC
                OS: All
            Status: NEW
          Severity: normal
          Priority: P
         Component: C++17
          Assignee: unassignedclangbugs at nondot.org
          Reporter: micjabbour at gmail.com
                CC: blitzrakete at gmail.com, erik.pilkington at gmail.com,
                    llvm-bugs at lists.llvm.org, richard-llvm at metafoo.co.uk

Given the following minimal example (which compiles successfully on GCC and
MSVC):


struct A {};

namespace detail {
// operator+ overload
A operator+(A, A) { return A{}; }
A f(A a) {
  return a + a; // this works fine
}
template <typename... As> A g(As... as) {
  // this however, fails to find operator+ when g is instantiated with "A"s
  return (as + ... + A{});
}
}

int main() {
  detail::f(A{}); // works
  detail::g(A{}); // fails!
}


Clang gives the following error:

D:\ws> clang++ -std=c++17 prog.cc
prog.cc:11:16: error: call to function 'operator+' that is neither visible in
the template definition nor found by argument-dependent lookup
  return (as + ... + A{});
               ^
prog.cc:17:11: note: in instantiation of function template specialization
'detail::g<A>' requested here
  detail::g(A{}); // fails!
          ^
prog.cc:5:3: note: 'operator+' should be declared prior to the call site or in
the global namespace
A operator+(A, A) { return A{}; }
  ^
1 error generated.

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