I have no clue what SCE_C_USERLITERAL is.
Its a C++ thing :)
A class can define a suffix to go after a literal to make it define an instance of the class instead of the type the literal usually makes. For example the `std::string` literal `"...."s` is actually just a user defined literal, not a built-in literal like the `"..."` char array literal is. For gory details see [here](http://en.cppreference.com/w/cpp/language/user_literal).
Unfortunately as that link above shows, it also applies to char and numeric literals, and Scintilla doesn't distinguish between them, dammit. But given that `std::string` literals are far more common than numeric literals (in my experience) I would still add it to strings until someone complains, then prod Neil with a patch to separate string user literals from the others.
Whatever QR and QW might mean.
Dunno, but there are other languages with them already added IIRC.