Visual Studio 2017 generates invalid code for caller.hpp

Under Python 3.6.5 and Visual Studio 2017 (15.7.2) and optimization level /O2 compilation of boost::python generates invalid code while compiling ".\boost\python\detail\caller.hpp". Namely, initialization of "static const signature_element ret" (line 245->) is erroneously skipped leading to SegFault in python.exe.

Reproduce the failure with Python 3.6.5, Visual Studio 2017 15.7.2, and /O2, then inspect boost/python/detail/caller.hpp around line 245. Compare optimized behavior where initialization of static const signature_element ret is skipped. Done means the generated code no longer causes the reported Python segmentation fault.

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