[Issue 18417] Make const and immutable postblit constructors illegal

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=18417

--- Comment #7 from Radu Racariu <radu.racariu at gmail.com> ---
As Steven Schveighoffer asked, how do you see an immutable refcounted struct
working after this change?

Consider this simplified example:
==============================================================
struct RC
{
  this (int i) immutable
  {
  }

  this(this)
  {
      import core.stdc.stdio;
      // prints `postblit called on RC`
      printf("postblit called on %s", typeof(this).stringof.ptr);
  }
}

void main()
{
    auto rc1 = immutable RC(1);
    auto rc2 = rc1;
    static assert(is(typeof(rc2) == immutable(RC)));
}
==============================================================

fixing this bug will imply that the postblit will not compile anymore for the
above example?

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