type of a variable as a mixin-able string?

[via Digitalmars-d-learn]

On Sunday, 21 June 2015 at 23:25:47 UTC, Timothee Cour wrote:
> Suppose I have:
>
> import std.range;
> auto a=iota(complex_expr_returning_3());
>
> I'd like to have a function/trait/template/compiler magic that 
> takes
> variable a and generates a string that can be mixed in to 
> represent the
> type of a. The difficulty is that typeid(a).to!string doesn't 
> work for
> Voldermort types:
> typeid(a).to!string produces "std.range.iota!(int, 
> int).iota.Result", which
> is not very useful as it can't be mixed in a program, being a 
> voldemort
> type, eg:
> mixin("std.range.iota!(int, int).iota.Result a;"); //compile 
> error
>
> However, the compiler could help reduce it to something as 
> simple as
> possible, eg:
> "typeof(iota(int.init))"
> here this would work:
>
> mixin("typeof(iota(int.init)) a;"); //works
>
> Is it possible to do that in a generic way, such that it hides 
> as much as
> possible the details of the expression (here, 
> "complex_expr_returning_3()"
> should be simplified to int.init)
>
> Or is there another trick I could use to instantiate a variable 
> with same type as a?
>
>
> NOTE:
> typeof(a) a2;
> works even though
> http://www.drdobbs.com/cpp/voldemort-types-in-d/232901591?pgno=2 says it
> won't work, "Sorry, that won't work, the compiler will not 
> allow a
> Voldemort Type to be instantiated outside of its scope "
> (although that's doesn't help with my original problem)

It's possibly not a real Voldemort type, i.e. one that closes 
over a local variable and therefore needs a context pointer. DMD 
correctly rejects this code:

     auto foo(int a) {
         struct S {
             int bar() {
                 return a;
             }
         }
         return S();
     }

     void main() {
         auto a = foo(42);
         typeof(a) b; // Error: cannot access frame pointer of 
xx.foo.S
     }