Error: @nogc function 'test.func2' cannot call non- at nogc delegate 'msg'

[Shachar Shemesh]

On 10/12/17 14:00, Shachar Shemesh wrote:
> On 10/12/17 13:44, Jonathan M Davis wrote:
>> it sounds like the delegate that's
>> being generated isn't @nogc, so it can't be called within the function,
>> which would be a completely different issue from allocating a closure.
> 
> Here's the thing, though. There is no reason for the delegate to be 
> called at all!
> 
> Since func1's msg is also lazy, I expected the lowering to look like this:
> 
> void func1(/*scope*/ string delegate() msg) @nogc {
> }
> 
> void func2(/*scope*/ string delegate() msg) @nogc {
>    func1(msg);
> }
> 
> Even with scope commented out, the above compiles just fine. The fact 
> that msg is not @nogc doesn't matter at all, simply because no one is 
> evaluating the delegate.

I think I got it. I think the lowering is this:

void func1(/*scope*/ string delegate() msg) @nogc {
}

void func2(/*scope*/ string delegate() msg) @nogc {
   func1((){ return msg(); });
}

With "scope" commented out, this generates the following error:
test.d(15): Error: function test.func2 is @nogc yet allocates closures 
with the GC
test.d(16):        test.func2.__lambda2 closes over variable msg at 
test.d(15)

So there are two problems here. The first is that a "lazy" generated 
delegate does not get forwarded to a function, and instead gets wrapped 
by a completely useless temporary delegate. The second is that scope on 
lazy is not honored.

I'll file a bug report.

Shachar