move-emulated swap causes overloaded ambiguous fun call

[st@…]

The attached test file is a minimal example of something I have a real-world use for (with T instead of int), and it doesn't compile in MS Visual Studio 2005 or 2008. The error message is too long as it's templated code, but the head follows: 1>c:\Users\dh\products\avir\include\boost/variant/detail/move.hpp(155) : error C2668: 'boost::detail::variant::detail::move_swap::swap' : ambiguous call to overloaded function 1> c:\Users\dh\products\avir\include\boost/variant/detail/move.hpp(141): could be 'void boost::detail::variant::detail::move_swap::swap<T>(T &,T &)' 1> with 1> [ 1> T=T2 1> ] 1> C:\Program Files (x86)\Microsoft Visual Studio 9.0\VC\include\utility(16): or 'void std::swap<T>(_Ty &,_Ty &)' [found using argument-dependent lookup] 1> with 1> [ 1> T=T2, 1> _Ty=T2 1> ] 1> while trying to match the argument list '(T2 , T2 )' 1> c:\Users\dh\products\avir\include\boost/variant/variant.hpp(684) : see reference to function template instantiation 'void boost::detail::variant::move_swap<T>(T &,T &)' being compiled 1> with 1> [ 1> T=T2 1> ]

The crucial code is in variant/detail/move.hpp, from line 114. As my variant includes a std::vector<T>*, it finds std::swap in the unqualified call on line 155.

The code in question is protected by a BOOST_WORKAROUND that disables it for gcc. I tested this case on gcc, and it works fine, but fails in the same way if I remove the workaround. The comment above it claims that it falls back for "non-conforming compilers," but I'm not convinced that the expected behaviour is conformant. Basic.lookup.koenig says explicitly that "If T is a pointer to U or an array of U, its associated namespaces and classes are those associated with U."

[st@…]

small test program

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MSVC output

[st@…]

Since the error message got wrapped anyway, I've added the whole output as an attachment.

[st@…]

patch adding test case to variant's unit tests

[Joseph.Gauterin]

We solved a similar problem in Boost.Swap by providing a templated swap with two template arguments [making it less specialized than std::swap]. I've had a quick look at variant/detail/move.hpp and I think it can be fixed in the same way.

[Laurent.Rineau@…]

The bug is still in Boost 1.53. Could a maintainer of Boost.Variant have a look at the test case and the proposed patch?

[Antony Polukhin]

(In [85927]) Optimize and fix ambiguity of move_swap in Boost.Variant (refs #2839) Fix GCC-4.8+ warning in variant_reference_test.cpp

[Antony Polukhin]

(In [86049]) Commit missing test file for Boost.Variant and uncomment line in Jamfile (refs #2839)

[Antony Polukhin]

(In [86650]) Merge from trunk:

• dropped support of antique compilers
• fixed issue with ambiguity in swap (fixes #2839)
• added conditional noexcepts to move assignments, default and move constructors (fixes #7911)
• experimental variadic templates support (refs #9163)
• bunch of size optimizations for assignments and move assignments (refs #7960)
• minor changes and size optimizations