I wrote a simple test case to demonstrate this:

    T test<T> (T param) {
        return param;
    }

    int main (string[] args) {
        test (4);
        test (Value (typeof (int)));
        return 0;
    }

The statement `test (4)` generates properly formed C code as expected, but using a GValue creates the following function call:

    _tmp2_ = test (G_TYPE_VALUE, (GBoxedCopyFunc) , g_value_unset, _tmp1_);

It emits the GBoxedCopyFunc cast but no expression to cast. Unsurprisingly, GCC doesn't like this much and chokes.