Unary plus (+) against literal string

Today I wrote an expression:

"<" + message_id + "@" +  + ">"
                          ^
                          |
                          \____  see that extra '+' here!

and got surprised that it actually compiled. (PS message_id is a QString, it would also work with an std::string)

I often do things like that, leave out a variable as I'm working and I expect the compiler to tell me where I'm still missing entries. The final would look something like this:

"<" + message_id + "@" + network_domain + ">"

Now I'd like to know why the + unary operator is valid against a string literal!?