import std.stdio;

struct S {string s; this(string x){s=x;}}

immutable(S) mkS(){
  return S("mks hi");
}

void main(char[][] args) {
  immutable(S) s = mkS();
  immutable(S)[] ss;
  ss ~= S("plain hi");
  ss ~= cast(immutable(S)) S("plain hi");
  //ss ~=  mkS(); // --> "Internal error: ../ztc/cod1.c 1684"
  writeln(s.s, ", ", ss[0].s);
}


I think (hope?) this is all valid code. I.e.,

  ss ~= S("Hi"); //should be ok
  ss.length = ss.length + 1;
  ss[$-1] = S("plain hi"); //should fail to compile (as it indeed does)

In any case there is an ICE.



 
Error: ss[0] isn't mutable
Error: ss[0] isn't mutable

  //ss[0] = S("fail"); --> Error: ss[0] isn't mutable