Compilation error when using Visual Studio 2017 with C++11/14/17 language support enabled

[AngusRo]

Using the latest version of toml11 the Visual Studio 2017 compiler generates the following error for a simple test case:

auto config = toml::parse("hello.toml");
auto devices = toml::find<int>(config, "device", 0);

Error generated:

include\toml11\parser.hpp(2682): error C2440: '': cannot convert from 'initializer list' to 'toml::detail::character_either'
1>include\toml11\parser.hpp(2834): note: No constructor could take the source type, or constructor overload resolution was ambiguous
1>include\toml11\parser.hpp(3729): error C2440: '': cannot convert from 'toml::error_info' to 'std::vector<toml::error_info,std::allocator<_Ty>>'
1> with
1> [
1> _Ty=toml::error_info
1> ]
1>include\toml11\parser.hpp(3729): note: No constructor could take the source type, or constructor overload resolution was ambiguous

[ToruNiina]

I changed code to make it compiles with MSVC 2017 (the original code compiles with gcc/clang, MSVC 2019, 2022 but only MSVC 2017 fails).

I added MSVC 2017 to appveyor build. It will continuously be checked after v4.3.0.