This renders default arguments in generic functions unusable.

void foo(U, T = int)(T v = T.init)
{
}

void main()
{
    foo!int(); // ok
    foo!int("x"); // fail
}

Error: function `foo` is not callable using argument types `(string)`

The second call tries to reuse the previous instantiation instead of creating a new one. An explicit instantiation works but defeats the purpose of using default arguments.