At:

https://developer.gnome.org/gtk3/stable/GtkInfoBar.html

In the infobar example (lines for context):

// set up info bar
GtkWidget *widget;
GtkInfoBar *bar;

widget = gtk_info_bar_new ();
bar = GTK_INFO_BAR (bar);

The assignment of:

bar = GTK_INFO_BAR (bar);      /* <--- ERROR 'bar' should be 'widget' */

is wrong and should be:

bar = GTK_INFO_BAR (widget);

To create the convenience reference cast of (GtkInfoBar *)widget;

Just needs fixing as it isn't something that takes long to figure out, but for someone working on infobar for the first time, it helps if the documentation is correct.