The compiler is allowed to assume that a boolean value passed as an argument is a valid boolean value (i.e. one which has been initialised or converted to true or false). The true value doesn't have to be the same as the integer 1 -- indeed, there can be various representations of true and false -- but the parameter must be some valid representation of one of those two values, where "valid representation" is implementation-defined.
So if you fail to initialise a bool, or if you succeed in overwriting it through some pointer of a different type, then the compiler's assumptions will be wrong and Undefined Behaviour will ensue. You had been warned:
50) Using a bool value in ways described by this International Standard as “undefined”, such as by examining the value of an uninitialized automatic object, might cause it to behave as if it is neither true nor false. (Footnote to para 6 of §6.9.1, Fundamental Types)