-----
module test;

// opaque type
struct S;

void main()
{
    S** s;
    void* ptr;
    *s = cast(S*)ptr;  // pointer assignment, not value assignment
}
-----

$ dmd test.d
> test.d(4): Error: struct test.S unknown size
> test.d(4): Error: struct test.S no size yet for forward reference
> test.d(4): Error: struct test.S unknown size
> test.d(4): Error: struct test.S no size yet for forward reference

The size of the structure does not need to be known since what is being assigned is a memory address. It is not a value assign of the struct.