Hovering a function pointer to a type that takes more arguments

As far as I know, casting between incompatible function pointers, for example:

void aUnaryFunction(int a1)
{
    /* .... */
}

void doSomethingWithFn()
{
    typedef void(*BinaryFn)(int, const char*);
    BinaryFn aBinaryFunction = (BinaryFn) &aUnaryFunction;
    aBinaryFunction (3, "!!!");
}

should never be executed, like "undefined behavior" according to the C standard.

However, I don’t understand why, given how function calls work in C, this example is unsafe. All I do is ignore the argument.

Assuming that the handling of the first int argument is consistent, all that will happen is that const char * will be put into the register when it doSomethingWithFn()calls aBinaryFunction, aUnaryFunctionwill be executed as expected, and const char * can be overwritten in time aUnaryFunction, but this is fine because nothing else will use it in any way.

- , ? ( - ?)