Typedef with and without struct name

Question

I saw the following oddity in the XCB API header. Most types are defined with a type alias and a struct name in this way:

typedef struct xcb_window_iterator_t {
    xcb_window_t *data;
    int rem;
    int index;
} xcb_window_iterator_t;

But some typedefs omit the struct name.

typedef struct {
    unsigned int sequence;
} xcb_void_cookie_t;

What does this mean from the C point of view?

Is the latter a type with a name and a struct without a name? Or is it no type but only a struct with a name?

My confusion comes from the output of the gccxml.

For xcb_window_iterator_t it generates a Struct node:

$ xmllint --format --xpath '//Struct[@name="xcb_window_iterator_t"]' xcb.xml

and a Typedef node:

$ xmllint --format --xpath '//Typedef[@name="xcb_window_iterator_t"]' xcb.xml

But for xcb_void_cookie_t it generates just a Struct node:

$ xmllint --format --xpath '//Struct[@name="xcb_void_cookie_t"]' xcb.xml

But no Typedef node:

$ xmllint --format --xpath '//Typedef[@name="xcb_void_cookie_t"]' xcb.xml 
XPath set is empty

Does this mean xcb_void_cookie_t is no type but just a struct, although the code contains the typedef? Or is it a bug in gccxml?