"The rules for conditional operator are quite sophisticated and one of them says that if one of the sub-expressions is a throw expression, the type of the entire conditional operator is that of the other sub-expression." [1]

[1] http://akrzemi1.wordpress.com/2011/05/06/compile-time-comput...

Thanks for both your and pdw's explanations...the lovely thing about C++ is that there's always something new to learn. ;)

It's valid in all versions of C++, I believe. It's usually motivated with a chained ternary operator:

  return a == ALPHA ? x :
         a == BETA ? y :
         a == GAMMA ? z :
         throw ...;

Although that's not a style I'd recommend.