The code below doesn't compile. Why this error message?
---
struct Item {
    int i;
}

struct Params {
    Item* item;

    this(int i) {
        item = new Item(i);  // line 9
    }
}

struct Foo(Params params) {}

enum foo = Foo!(Params(1));
---

test.d(9): Error: Item(1) is not an lvalue

On Thursday, 2 April 2015 at 23:12:25 UTC, biozic wrote:
> The code below doesn't compile. Why this error message?
> ---
> struct Item {
>     int i;
> }
>
> struct Params {
>     Item* item;
>
>     this(int i) {
>         item = new Item(i);  // line 9
>     }
> }
>
> struct Foo(Params params) {}
>
> enum foo = Foo!(Params(1));
> ---
>
> test.d(9): Error: Item(1) is not an lvalue

this doesn't work because struct Foo is parameterised by in instance of Params. (like if you were implementing a fixed size array: struct FixedSizeArray(size_t len) {...}. here FixedSizeArray is parameterised by in instance of a size_t).

Item is a pointer in Params and thus if it were to compile the compiler would need to know the value returned by `new`at compile time.

There are two ways to get that to compile change `Item* item;` to `Item item;` or change `item = new Item(1);` to `item = null;`

tl;dr you can't have r-value (pointers) in instance template parameter lists.
i.e. struct foo(my_reference_type_or_type_containg_pointers instance) { ... }