Answer 1

Very interesting UB ¹!

The compilation log:

"warning: operation on 'a' may be undefined [-Wsequence-point]"

Reason:

"When modifying an object between two sequence points, reading the value of the object for any other purpose than determining the value to be stored is also undefined behavior."

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¹ https://en.wikipedia.org/wiki/Undefined_behavior#Examples_in_C_and_C++

Answer 2

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