The implicit version tuple's "reduced-arity-initialization" extension breaks conforming code.

[llvmbot]

Bugzilla Link	27374
Resolution	FIXED
Resolved on	Dec 08, 2016 17:58
Version	unspecified
OS	All
Reporter	LLVM Bugzilla Contributor
CC	@mclow

Extended Description

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
#include

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.

[llvmbot]

Fixed in r289158 by disabling the extension on the implicit constructors. To support existing users of the extension it can be re-enabled using a macro.