Note that it's not reported with gcc. It's only reported with g++.

C++ code to reproduce this warning:

    #include <glib-object.h>

    G_BEGIN_DECLS

    #define GARROW_TYPE_FILE (garrow_file_get_type())
    G_DECLARE_INTERFACE(GArrowFile,
                        garrow_file,
                        GARROW,
                        FILE,
                        GObject)

    struct _GArrowFileInterface {
      GTypeInterface g_iface;
    };

    G_DEFINE_INTERFACE(GArrowFile,
                       garrow_file,
                       G_TYPE_OBJECT)

    static void
    garrow_file_default_init(GArrowFileInterface *iface)
    {
    }

    G_END_DECLS

Build command line:

    % g++ -Wall -shared -o liba.so a.cpp $(pkg-config --cflags --libs gobject-2.0)

Message:

    In file included from /tmp/local.glib/include/glib-2.0/gobject/gobject.h:24,
                     from /tmp/local.glib/include/glib-2.0/gobject/gbinding.h:29,
                     from /tmp/local.glib/include/glib-2.0/glib-object.h:23,
                     from a.cpp:1:
    a.cpp: In function 'GType garrow_file_get_type()':
    /tmp/local.glib/include/glib-2.0/gobject/gtype.h:219:50: warning: '<<' in boolean context, did you mean '<' ? [-Wint-in-bool-context]
     #define G_TYPE_MAKE_FUNDAMENTAL(x) ((GType) ((x) << G_TYPE_FUNDAMENTAL_SHIFT))
                                                 ~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~
    /tmp/local.glib/include/glib-2.0/gobject/gtype.h:2026:11: note: in definition of macro '_G_DEFINE_INTERFACE_EXTENDED_BEGIN'
           if (TYPE_PREREQ) \
               ^~~~~~~~~~~
    /tmp/local.glib/include/glib-2.0/gobject/gtype.h:1758:47: note: in expansion of macro 'G_DEFINE_INTERFACE_WITH_CODE'
     #define G_DEFINE_INTERFACE(TN, t_n, T_P)      G_DEFINE_INTERFACE_WITH_CODE(TN, t_n, T_P, ;)
                                                   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
    a.cpp:16:1: note: in expansion of macro 'G_DEFINE_INTERFACE'
     G_DEFINE_INTERFACE(GArrowFile,
     ^~~~~~~~~~~~~~~~~~
    /tmp/local.glib/include/glib-2.0/gobject/gtype.h:178:25: note: in expansion of macro 'G_TYPE_MAKE_FUNDAMENTAL'
     #define G_TYPE_OBJECT   G_TYPE_MAKE_FUNDAMENTAL (20)
                             ^~~~~~~~~~~~~~~~~~~~~~~
    a.cpp:18:20: note: in expansion of macro 'G_TYPE_OBJECT'
                        G_TYPE_OBJECT)
                        ^~~~~~~~~~~~~