std::wstring temp = std::transform( field->GetFieldType().begin(), field->GetFieldType().end(), field->GetFieldType().begin(), [](wchar_t c) { return std::towlower( c ); } );
if( temp == L"date" || temp == L"text" )

Doing this in MSVC produces:

1\>  c:\\users\\igor\\onedrive\\documents\\dbhandler_app\\libsqlite\\database_sqlite.cpp(1227): error C2440: 'initializing': cannot convert from '\_OutIt' to 'std::basic_string\,std::allocator\\>'
1\>          with
1\>          \[
1\>              \_OutIt=std::\_String_const_iterator\\>\>
1\>          \]
1\>  c:\\users\\igor\\onedrive\\documents\\dbhandler_app\\libsqlite\\database_sqlite.cpp(1227): note: No constructor could take the source type, or constructor overload resolution was ambiguous

Using auto instead of std::wstring gives different problem:

1\>  c:\\users\\igor\\onedrive\\documents\\dbhandler_app\\libsqlite\\database_sqlite.cpp(1228): error C2678: binary '==': no operator found which takes a left-hand operand of type '\_OutIt' (or there is no acceptable conversion)
1\>          with
1\>          \[
1\>              \_OutIt=std::\_String_const_iterator\\>\>
1\>          \]
1\>  c:\\program files (x86)\\microsoft visual studio\\2017\\community\\vc\\tools\\msvc\\14.16.27023\\include\\xstring(1555): note: could be 'bool std::\_String_const_iterator\\>\>::operator ==(const std::\_String_const_iterator\\>\> &) const'
1\>          with
1\>          \[
1\>              \_Ty=wchar_t
1\>          \]
1\>  c:\\program files (x86)\\windows kits\\10\\include\\10.0.10240.0\\shared\\guiddef.h(192): note: or       'bool operator ==(const GUID &,const GUID &)'
1\>  c:\\program files (x86)\\microsoft visual studio\\2017\\community\\vc\\tools\\msvc\\14.16.27023\\include\\exception(321): note: or       'bool std::operator ==(const std::exception_ptr &,const std::exception_ptr &) noexcept' \[found using argument-dependent lookup\]
1\>  c:\\program files (x86)\\microsoft visual studio\\2017\\community\\vc\\tools\\msvc\\14.16.27023\\include\\exception(326): note: or       'bool std::operator ==(std::nullptr_t,const std::exception_ptr &) noexcept' \[found using argument-dependent lookup\]
1\>  c:\\program files (x86)\\microsoft visual studio\\2017\\community\\vc\\tools\\msvc\\14.16.27023\\include\\exception(331): note: or       'bool std::operator ==(const std::exception_ptr &,std::nullptr_t) noexcept' \[found using argument-dependent lookup\]
1\>  c:\\program files (x86)\\microsoft visual studio\\2017\\community\\vc\\tools\\msvc\\14.16.27023\\include\\system_error(266): note: or       'bool std::operator ==(const std::error_code &,const std::error_code &) noexcept' \[found using argument-dependent lookup\]
1\>  c:\\program files (x86)\\microsoft visual studio\\2017\\community\\vc\\tools\\msvc\\14.16.27023\\include\\system_error(272): note: or       'bool std::operator ==(const std::error_code &,const std::error_condition &) noexcept' \[found using argument-dependent lookup\]
1\>  c:\\program files (x86)\\microsoft visual studio\\2017\\community\\vc\\tools\\msvc\\14.16.27023\\include\\system_error(278): note: or       'bool std::operator ==(const std::error_condition &,const std::error_code &) noexcept' \[found using argument-dependent lookup\]
1\>  c:\\program files (x86)\\microsoft visual studio\\2017\\community\\vc\\tools\\msvc\\14.16.27023\\include\\system_error(284): note: or       'bool std::operator ==(const std::error_condition &,const std::error_condition &) noexcept' \[found using argument-dependent lookup\]
1\>  c:\\program files (x86)\\microsoft visual studio\\2017\\community\\vc\\tools\\msvc\\14.16.27023\\include\\thread(208): note: or       'bool std::operator ==(std::thread::id,std::thread::id) noexcept' \[found using argument-dependent lookup\]
1\>  c:\\users\\igor\\onedrive\\documents\\dbhandler_app\\libsqlite\\database_sqlite.cpp(1228): note: or       'built-in C++ operator==(const wchar_t \[5\], const wchar_t \[5\])'
1\>  c:\\users\\igor\\onedrive\\documents\\dbhandler_app\\libsqlite\\database_sqlite.cpp(1228): note: while trying to match the argument list '(\_OutIt, const wchar_t \[5\])'
1\>          with
1\>          \[
1\>              \_OutIt=std::\_String_const_iterator\\>\>
1\>          \]

How to convert and return?

Don't care about locales - just need a type conversion.