c - Any guarantees for uninitialised variables?

Question

There are many claims that any use of uninitialised variables invokes undefined behavior (UB).
Perusing the docs, I could not verify that claim, so I would like a convincing argument clarifying this for both C and C++.
I expect the same semantics for both, but am prepared to be surprised by subtle or not so subtle differences.

Some examples of using uninitialised variables to get started. Please add others as needed to explain any corner-cases they don't cover.

void test1() {
    int x;
    printf("%d", x);
}

void test2() {
    int x;
    for(int i = 0; i < CHAR_BIT * sizeof x)
        x = x << 1;
    printf("%d", x);
}

void test3() {
    unsigned x;
    printf("%u", x); /* was format "%d" */
}

void test4() {
    unsigned x;
    for(int i = 0; i < CHAR_BIT * sizeof x)
        x = x << 1;
    printf("%u", x); /* was format "%d" */
}

Answer 1

In C all of them are undefined behavior, but for a reason that probably not comes directly to mind. Accessing an object with indeterminate value has undefined behavior if it is "memoryless" that is 6.3.2.1 p2

If the lvalue designates an object of automatic storage duration that could have been declared with the register storage class (never had its address taken), and that object is uninitialized (not declared with an initializer and no assignment to it has been performed prior to use), the behavior is undefined.

Otherwise, if the address is taken, the interpretation of what indeterminate means concretely in this case is not unanimous. There are people that expect such a value to be fixed once it is first read, others speak of something like "woobly" (or so) values that can be different at each access.

In summary, don't do it. (But that you probably knew already.)

(And not talking about the error using "%d" for an unsigned.)