subtlety or bug?

[Carl Sturtivant via Digitalmars-d-learn]

The following compiles and runs correctly.
https://forum.dlang.org/post/tzwsohkcqrkqotbwnhjb@forum.dlang.org

But if I add a destructor to the reference struct template as 
follows, it no longer compiles, and the complaints are not about 
the destructor.
```
     ~this()
     {
         ptr = null;
     }
```
refsim.d(39): Error: generated function 
refsim.reference!int.reference.opAssign (reference!int p) is not 
callable using argument types (int)
refsim.d(42): Error: generated function 
refsim.reference!int.reference.opAssign (reference!int p) is not 
callable using argument types (int)

These are complaining about lines where reference!int variables 
are assigned integers.

What's going on?

Here's the code linked to above without the destructor that works.
```
struct reference(T)
{
     T* ptr;
     this(ref T x)
     {
         ptr = &x;
     }
     import std.exception : enforce;

     ref T cnvrt() @property
     {
         enforce( ptr !is null);
         return *ptr;
     }
     ref T cnvrt(T x) @property
     {
         enforce( ptr !is null);
         return *ptr = x;
     }
     alias cnvrt this;
}

void main()
{
     int i;
     auto ri = reference!int(i);
     auto ri2 = reference!int(ri);

     assert(ri.ptr==ri2.ptr);

     i = 99;
     assert(i==ri && i==ri2 && ri==ri2);

     ri = 100;
     assert(i==ri && i==ri2 && ri==ri2);

     ri2 = 101;
     assert(i==ri && i==ri2 && ri==ri2);
}
```