Safe cast away from immutable

[Iakh via Digitalmars-d]

On Monday, 8 February 2016 at 21:48:30 UTC, Jonathan M Davis 
wrote:

> that right now, but clearly, what it currently has is buggy,

Yeah. Looks like it just traverse params's AST and search for
exactly match with ReturnType.

The code with replaced (void, int) with (class A, class B : A)
behaves the same way as original:

import std.stdio;

class A
{
     int i;
}

class B : A
{
}

struct S
{
     A a;

     auto f() pure @safe
     {
         B b = new B;
         a = b;
         return b;
     }
}

void main() @safe
{
     S s;
     immutable a = s.f();
     A b = s.a;
     writeln(a.i);
     b.i = 1;
     writeln(a.i);
}