This is a bug with the gagging system.

auto quz(X)(X y)
{
    X q = "abc";
    return y;
}

static assert(!is(typeof(quz!int(6))));

Two additional tests:
// It should also be false the second time;
static assert(!is(typeof(quz!int(18))));
// This should print error messages.
int bolog = quz!int(27);