[Boost-users] Ambiguous overload for boost::blank when using optional & variant

Subject: [Boost-users] Ambiguous overload for boost::blank when using optional & variant
From: Berteun Damman (berteun_at_[hidden])
Date: 2015-01-14 07:09:47

Hi,

I'm using boost 1.57 after upgrading from 1.50, and the following
innocuous looking program does not compile with either GCC (4.9) or
Clang (3.5); tested both on Mac & Linux:

#include <boost/optional.hpp> // This header triggers the problem
#include <boost/variant.hpp>

int main() {
  boost::variant<boost::blank> a, b;
  swap(a,b); // a.swap(b) does not make a difference
}

Both  GCC and Clang give the same error:

/opt/local/include/boost/variant/detail/move.hpp:60:5: error: call to
'swap' is ambiguous
    swap(lhs, rhs);

[....]

/opt/local/include/boost/core/swap.hpp:54:8: note: candidate function
[with T1 = boost::blank, T2 = boost::blank]
  void swap(T1& left, T2& right)
       ^

/opt/local/include/boost/variant/detail/move.hpp:44:13: note:
candidate function [with T1 = boost::blank, T2 =boost::blank]

inline void swap(T1& lhs, T2& rhs)

In this case one could remove the Optional header, but in a larger
project this is of course not a solution; a better solution seems to
add an overload for swap taking two boost blanks.

I feel this is a bug in Boost, can someone confirm and tell me where I
should file the bug request?

Finally I noticed due to the above error messages that boost::variant
will never use variadic templates when you use Clang. For GCC it uses
them (in C++11 mode), but for Clang it refuses. Why is this?

Thanks,

-Berteun