Error: field r must be initialized in constructor, because it is nested struct

[anonymous via Digitalmars-d-learn]

On Wednesday, 3 June 2015 at 23:27:31 UTC, Ali Çehreli wrote:
> Pretty standard thing doesn't work and I can't find it on 
> bugzilla:
>
> import std.algorithm;
>
> struct Foo(R)
> {
>     R r;
>
>     this(R r)    // <-- Compilation error
>     {}
> }
>
> auto foo(R)(R r)
> {
>     return Foo!R(r);
> }
>
> void main()
> {
>     auto arr = [1];
>     arr.map!(i => i).foo;
> }

You can shut the compiler up by doing `this.r = R.init;` in the 
constructor.

Searching for "must be initialized" yields issue 13945 which 
calls for better documentation: 
https://issues.dlang.org/show_bug.cgi?id=13945

Not covered there is why `map!(i => i)` results in a nested 
struct. This is the compiler being conservative/stupid, I guess. 
The lambda could require a context pointer. It doesn't, but the 
compiler isn't smart enough to see that.

Some variants that don't trip the compiler up:

arr.map!((int i) => i).foo;

static auto identity(T)(T x) {return x;}
arr.map!identity.foo;

arr.map!"a"

It would be nice if the compiler would take this hint, but it 
doesn't:

arr.map!(function (i) => i).foo;