Putting all the maintainability and reading issues aside, can these lines of code generate undefined behavior?
float a = 0, b = 0;
float& x = some_condition
This is absolutely fine, as long as both sides of the conditional are expressions that can be used to initialize a reference (e.g. variables, pointer dereferences, etc)
float& x = some_condition()? a : *(&b); // This is OK - it is the same as your code
float& x = some_condition()? a : b+1; // This will not compile, because you cannot take reference of b+1