Following code compilation crashes dmd.

import std.typecons;
void main()
{
    Nullable!(int, 0) ni;
    ni = ni;
}

This is alias this issue that fallbacks to template member function.
Reduced test case:

struct S(T)
{
    void opAssign()(T value) {}
}
struct X(T)
{
    private T val;
    @property ref inout(T) get()() inout { return val; }
    alias get this;
}
void test()
{
    S!(int) s;
    X!int x;
    s = x;
}