Well for me it's the same reason I don't give functions names that suggest something different from what they do.
boolean_flag
? do_this()
: do_that();
The only possibly feasible way i see for it doing something unexpected would be if it were the last statement in a function in a language that implicitly returns the value returned by the last statement in a function; but users of such languages usually are aware that can happen.
How do they suggest that?
As far as i am aware ternaries put no restriction whatsoever on what kind of code you can put in its branches.
I don't buy the comparison with getX either, since ternaries have no name, they're just some punctuation, and even if implemented as if/else word pairs are completely vague.
You just called someone out for this:
"If you're restricting what you're saying to a specific language say that bold and as the very first thing (...)"
Are you talking about a specific language now? Because some languages do put restrictions on that. Python makes a clear distinction between statements and expressions, so you don't even get the option to use a ternary in place of an if/else block.
That said, (in general) I disagree that ternaries suggest side-effect free. However, they do suggest that you care about the return value of the ternary expression. Some languages enforce this suggestion, others don't. But even for languages that don't, a ternary that silently discards its return value feels like a "cute hack" to me.
It might be a convention in some circles. I don't think it's a well-established one, though.
For me, ternaries strongly suggest "expressions being run for its return value", but without any indication whether or not it's side effect-free. In practice, most of the time such expressions end up being side effect-free (getters, finders, etc.), but every now and then you do want to have a stateful expression (e.g. getting something from a map, and storing a default value there if the key is missing in the map). I believe such stateful expressions are fine to use with ternaries as well.