EDIT: It's syntactically legal, but results in undefined behavior if you use x.
It's not legal because you're assigning an uninitialized variable with another (well, the same) uninitialized variable. Just because it compiles doesn't mean it's legal. It's valid C++ syntax, yes, but not legal.
The right hand side of the assignment operator must be fully evaluated at the time of the assignment. In this case, that's i, which isn't initialized.
Credits to Steve Jessop, who dug up the quote:
4.1/1, lvalue-to-rvalue conversion
[...] if the object is uninitialized, a program that necessitates this
conversion has undefined behavior.
