Standard writes in 8.5.4.1: List-initialization
Note: List-initialization can be used
- as the initializer in a variable definition (8.5)
- as the initializer in a new expression (5.3.4)
- in a return statement (6.6.3)
- as a function argument (5.2.2)
- as a subscript (5.2.1)
- as an argument to a constructor invocation (8.5, 5.2.3)
- as an initializer for a non-static data member (9.2)
- in a mem-initializer (12.6.2)
- on the right-hand side of an assignment (5.17)
Nothing of them is a ternary operator. The more minimalistic return 1?{}:{}; is invalid too, what you want is impossible.
Of course you can explicitly call the constructor std::list<std::string>{}, but I would recommend to write out the if–else-block as you already did.