Casting subclass to parent of three class heirarchy causes crashes

The following shader which attempts to cast a variable of type C to its immediate parent, type B while passing an argument crashes DXC. If the only classes are B and C, this works fine. If there is a parent class to B, then it fails. Passing the argument as class C works fine as well.

struct A {
  float4 stuff;
};
struct B : A {
  float4 gimme() {return stuff;}
};
struct C : B {
  void dostuff() {stuff = 0;}
};
float4 f(B thing1) { // THIS IS THE PROBLEM
  return thing1.gimme();
}
float4 main() : SV_Target
{
  C thing2;
  thing2.stuff = float4(1,2,3,4);
  return f(thing2);
}

Start by compiling the provided shader with DXC and use main plus f(B thing1) as the reproduction entry points. Trace the C-to-B argument conversion through the A/B/C hierarchy; done means this case no longer crashes and returns the initialized value.