I found this bug while std.typecons.Rebindable checking.

Test cases.
----
class C {}
class D : C {}

struct S { C c; alias c this; } // S : C
struct T { S s; alias s this; } // T : S
struct U { T t; alias t this; } // U : T

void main()
{
    C c;
    D d;
    S s;
    T t;
    U u;

    assert(c is c);  // OK
    assert(c is d);  // OK
    assert(c is s);  // OK
    assert(c is t);  // OK
    assert(c is u);  // OK

    assert(d is c);  // OK
    assert(d is d);  // OK
    assert(d is s);  // doesn't work
    assert(d is t);  // doesn't work
    assert(d is u);  // doesn't work

    assert(s is c);  // OK
    assert(s is d);  // doesn't work
    assert(s is s);  // OK
    assert(s is t);  // doesn't work
    assert(s is u);  // doesn't work

    assert(t is c);  // OK
    assert(t is d);  // doesn't work
    assert(t is s);  // doesn't work
    assert(t is t);  // OK
    assert(t is u);  // doesn't work

    assert(u is c);  // OK
    assert(u is d);  // doesn't work
    assert(u is s);  // doesn't work
    assert(u is t);  // doesn't work
    assert(u is u);  // OK
}