Initializer with side-effect

Question

The C18 Standard states at 6.7.9/2 that:

No initializer shall attempt to provide a value for an object not contained within the entity being initialized.

It is not really clear what it means. There is a related topic: Incosistent gcc diagnostic for string initialization. The clause I cited was used to explain the error produced by the following initialization:

//error: excess elements in array initializer char a[5]
char a[5] = {'h','e','l','l','o','\0'}; 

Where the initializer-list has length exceeding the size of the array being initialized.

But consider more trivial example:

int main(void){
    int a;
    int b = (a = 3);
}

The initializer here is (a = 3) which is an assignment-expression. And the initializer assigns a value to another object which should cause constraint-violation.

Why isn't any diagnostic printed?

Answer 1

That's not "providing a value for a". It's an expression with a side effect of storing a value in a. "Providing a value" should be understood in the context of the initialization; it's not phrasing used for assignments.

A less trivial example of what the language in the standard is trying to get at might be:

struct {
    char a[4];
    char b[4];
} foo = { { 'h', 'e', 'l', 'l', 'o' } };

whereas (I think; correct me if I'm wrong) the following would be valid, albeit confusing, C:

struct {
    char a[4];
    char b[4];
} foo = { 'h', 'e', 'l', 'l', 'o' };