Accessing mutable data that isn't

[qznc]

On Wednesday, 20 November 2013 at 22:49:42 UTC, Spott wrote:
> I've been screwing around with templates lately, and I'm
> attempting to figure out why the following won't compile:
>
> struct value
> {
>      int a;
>
>      const auto
>          opBinary(string op, T)(in T rhs) const pure {
>              static if (op == "+")
>                  return 
> intermediateValue!(value.plus,this,rhs)();
>          }
>
>      ref value opAssign(T)( in T t ) {
>          a = t.a;
>          return this;
>      }
>
>      static
>      int plus(T1, T2)(in T1 x, in T2 y) pure {
>          return x.a + y.a;
>      }
>
> }
>
> struct intermediateValue(alias Op, alias A, alias B)
> {
>
>      auto opBinary(string op, T)(in T rhs) const pure {
>          static if (op == "+")
>              return intermediateValue!(value.plus,this,rhs)();
>      }
>
>      @property auto a() const pure {
>          return Op(A, B);
>      }
>
> }
>
> void main()
> {
>      value a = value(2);
>      value b = value(3);
>      value c;
>      c = a + b;
> }
>
> The error is:
> d_playground.d(34): Error: pure nested function 'a' cannot 
> access
> mutable data 'this'
> d_playground.d(34): Error: pure nested function 'a' cannot 
> access
> mutable data 'this'
> d_playground.d(10): Error: template instance
> d_playground.value.opBinary!("+",
> value).opBinary.intermediateValue!(plus, this, rhs) error
> instantiating
> d_playground.d(44):        instantiated from here: 
> opBinary!("+",
> value)
> d_playground.d(44): Error: template instance
> d_playground.value.opBinary!("+", value) error instantiating
>
> What is going on?  Why is 'a' not allowed to "access" mutable
> data (even though it isn't modifying it)? How do I tell the
> compiler to pass "this" in a const fashion?

No answer, but two notes.

First, use dpaste for such code snippets: 
http://dpaste.dzfl.pl/f2f39b32

Second, what are you trying to do? intermediateValue is a struct 
without members. I am not sure what 'this' means in such a case.