Compiler has the freedom to use assumption that the code will never hit undefined behavior. To actually access the variable you have to hit undefined behavior.
Actually the compiler (well, the author thereof) must know that the code may very well hit undefined behavior, and address how such known situations should be handled.
behavior, such as might arise upon use of an erroneous program construct or erroneous data, for which this International
Standard imposes no requirements. Undefined behavior may also be expected when this International Standard omits
the description of any explicit definition of behavior. [ Note: permissible undefined behavior ranges from ignoring the
situation completely with unpredictable results, to behaving during translation or program execution in a documented
manner characteristic of the environment (with or without the issuance of a diagnostic message), to terminating a translation
or execution (with the issuance of a diagnostic message). Many erroneous program constructs do not engender
undefined behavior; they are required to be diagnosed. —end note ]
1.3.14 unspecified behavior [defns.unspecified]
behavior, for a well-formed program construct and correct data, that depends on the implementation. The implementation
is not required to document which behavior occurs.
The spec goes on to explicitly state, over 150 times, certain situations result in undefined behavior; my favorite being:
Using a bool value in ways described by this International Standard as “undefined,” such as by examining the value of an uninitialized automatic
variable, might cause it to behave as if it is neither true nor false.
