[llvm-bugs] [Bug 44034] New: Clang++ accepts invalid template code

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=44034

            Bug ID: 44034
           Summary: Clang++ accepts invalid template code
           Product: clang
           Version: trunk
          Hardware: PC
                OS: Windows NT
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: C++
          Assignee: unassignedclangbugs at nondot.org
          Reporter: Darrell.Wright at gmail.com
                CC: blitzrakete at gmail.com, dgregor at apple.com,
                    erik.pilkington at gmail.com, llvm-bugs at lists.llvm.org,
                    richard-llvm at metafoo.co.uk

I accidently produced this invalid code and it worked as intended until I
tested further.  However, the integer_sequence should never have been deduced
https://gcc.godbolt.org/z/dn5s4v  

Affects older clang too.

This will return 20, but should be an error 

#include <algorithm>
#include <type_traits>

namespace algorithm_impl {
template <typename Function, typename... Args, size_t... Is>
constexpr void do_n(Function &&func, Args &&... args,
                    std::integer_sequence<size_t, Is...>) {
  if constexpr (sizeof...(Is) > 0) {
    (void)((func(args...), Is) + ...);
  }
}
}  // namespace algorithm_impl

template <size_t count, typename Function, typename... Args>
constexpr void do_n(Function &&func,
                    Args &&... args) noexcept(noexcept(func(args...))) {
  algorithm_impl::do_n(std::forward<Function>(func),
                       std::forward<Args>(args)...,
                       std::make_integer_sequence<size_t, count>{});
}

constexpr int do_n_1_test() {
  int n = 0;
  do_n<20>([&n] { ++n; });
  return n;
}

int main() { return do_n_1_test(); }

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