The following snipped fails to compile. What seems to happen is that the return statement implicitly calls the constructor which in turn returns a mutable S which is not implicitly castable to immutable - thus the error.

Happens since DMD 2.054

---
struct S {
	int[] c;
	this(in S){}
	immutable(S) f() const {
		S s;
		return cast(immutable)s;
		// the following works:
		//immutable(S) sr = cast(immutable)s;
		//return sr;
	}
}
---

dbug2.d(6): Error: cannot implicitly convert expression ((immutable immutable(S) __ctmp3 = 0;
 , __ctmp3).this(s)) of type S to immutable(S)


.. leaving the severity as normal, as there is an easy workaround