Can a variable be used when reporting?

Why does the following compile without an error?:

int main()
{
    int x = x;  //I thought this should cause an error
    return 0;
}

Where in the standards is it explained why this is allowed?

This question has a slightly different answer in C than in C++.

In both cases, int x = x; tries to initialize x with itself.

In C++: [dcl.init]/12 (N3936) says that any evaluation of an object with indeterminate value causes undefined behaviour, except for certain cases involving unsigned char. In fact there is an Example:

int f(bool b) {
    unsigned char c;
    unsigned char d = c; // OK, d has an indeterminate value
    int e = d;        // undefined behavior
    return b ? d : 0; // undefined behavior if b is true
}

In C: It is more complicated. This is very similar to the behaviour of int b; foo(b - b); which is covered in full here.

I won't repeat that text but the conclusions are, in C11:

• int a = a; &a; causes UB if and only if the system has trap representations for int
• int a = a;, with no subsequent occurrence of &a, causes UB.

Historical note: In C90 this caused UB. In C99 the trap representation was introduced, and in C11 the possibility of register trap was introduced (for Itanium). The C++ standard doesn't deal with trap representations at all, it seems underspecified in the case of things like bitwise operators generating negative zero.