It seems that the latest develop branch has 'stdext::hash_value' compile failure

[WadeGao]

I found that the latest develop branch(commit id: 8e20aa1) will generate compile failures because of std::hash related error when using below CI options on Appveyor:

• Environment: FLAVOR=Visual Studio 2017 C++2a Strict, APPVEYOR_BUILD_WORKER_IMAGE=Visual Studio 2017, B2_CXXFLAGS=-permissive-, B2_CXXSTD=2a, B2_TOOLSET=msvc-14.1
• Environment: FLAVOR=Visual Studio 2017 C++14/17, APPVEYOR_BUILD_WORKER_IMAGE=Visual Studio 2017, B2_CXXSTD=14,17, B2_TOOLSET=msvc-14.1
• Environment: FLAVOR=clang-cl, APPVEYOR_BUILD_WORKER_IMAGE=Visual Studio 2017, B2_ADDRESS_MODEL=64, B2_CXXSTD=11,14,17, B2_TOOLSET=clang-win
• Environment: FLAVOR=Visual Studio 2015, 2013, APPVEYOR_BUILD_WORKER_IMAGE=Visual Studio 2015, B2_TOOLSET=msvc-12.0,msvc-14.0
• Environment: FLAVOR=Visual Studio 2008, 2010, 2012, APPVEYOR_BUILD_WORKER_IMAGE=Visual Studio 2015, B2_TOOLSET=msvc-9.0,msvc-10.0,msvc-11.0, B2_ADDRESS_MODEL=32

To regenerate this issues, I create a redundant draft pull request by adding only single line comment (sorry for my unruly behavior), but the failures still occur, details can be found in this links.

The same failure has affected several recent pull requests

Failure digest:

.\boost/container_hash/hash.hpp(669): error C2668: 'stdext::hash_value': ambiguous call to overloaded function
C:\Program Files (x86)\Microsoft Visual Studio\2017\Community\VC\Tools\MSVC\14.16.27023\include\xhash(29): note: could be 'size_t stdext::hash_value<T>(const _Kty &)' [found using argument-dependent lookup]
        with
        [
            T=std::vector<unsigned long,std::allocator<unsigned long>>,
            _Kty=std::vector<unsigned long,std::allocator<unsigned long>>
        ]
.\boost/container_hash/hash.hpp(406): note: or       'unsigned int boost::hash_value<T>(const T &)'
        with
        [
            T=std::vector<unsigned long,std::allocator<unsigned long>>
        ]
.\boost/container_hash/hash.hpp(669): note: while trying to match the argument list '(const T)'
        with
        [
            T=std::vector<unsigned long,std::allocator<unsigned long>>
        ]