[llvm-bugs] [Bug 27374] New: The implicit version tuple's "reduced-arity-initialization" extension breaks conforming code.

[via llvm-bugs]

https://llvm.org/bugs/show_bug.cgi?id=27374

            Bug ID: 27374
           Summary: The implicit version tuple's
                    "reduced-arity-initialization" extension breaks
                    conforming code.
           Product: libc++
           Version: unspecified
          Hardware: PC
                OS: All
            Status: NEW
          Severity: normal
          Priority: P
         Component: All Bugs
          Assignee: unassignedclangbugs at nondot.org
          Reporter: eric at efcs.ca
                CC: llvm-bugs at lists.llvm.org, mclow.lists at gmail.com
    Classification: Unclassified

This bug tracks LWG issue #2419
(http://cplusplus.github.io/LWG/lwg-active.html#2419)

Calls to foo(v) can implicitly construct a n-tuple where n > 1 due to libc++'s
tuple extensions. This can cause overload resolution for foo(v) to select a
different overload than would otherwise be choosen for a conforming tuple
implementation. For example:

#include <iostream>
#include <tuple>

struct base
{
  void out(const std::tuple<char, char>& w) const
  {
    std::cerr << "Tuple: " << std::get<0>(w) << std::get<1>(w) << '\n';
  }
};

struct decorator
{
  base b_;

  template <typename... Args>
  auto
  out(Args&&... args)
    -> decltype(b_.out(args...))
  {
    return b_.out(args...);
  }

  void out(const char& w)
  {
    std::cerr << "char: " << w << '\n';
  }
};

int main()
{
  decorator d{base{}};
  char l = 'a';
  d.out(l);
}

Libc++'s tuple will cause this program to print "Tuple: a" where it should
actually print "char: a".

The fix for this problem is to remove the extension on the implicit version of
the UTypes... constructors.

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