MSVC uses wrong field type for strings

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MSVC seems to use bool variant instead of std::string for const char*.

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This is a bug, eventually fixed in C++20 (VS 2019 16.10):

• https://stackoverflow.com/questions/60681710/prefer-stdstring-in-stdvariantbool-stdstring-for-const-char
• https://www.cppstories.com/2018/06/variant/#c20--c23-improvements
• https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p0608r3.html

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Workaround implemented. It's no longer necessary once C++20 is used.