Answer 1

Always avoid your code with such types of calculation it may cause UB some compiler will give you warnings and its Compiler dependent . And always use different different variables so it will calculate more accurate . if you will try these calculations on java it may give you errors.

Answer 2

The compiler is confused just like I am.
The compiler is confused whether to perform addition or increment or both. So it assumed it to be undefined in simple words. (schrodinger effect) ;)

Who would write such a confusing code anyway?