Hi,

I just stumbled over this behavior. I am not sure whether
the behavior is correct or not.

alias foo = () => new Object;
void bar(Object o){}

void main()
{
	auto n1 = foo;
	bar(foo);
}

While first line in main is working fine,
second line does not compile due to missing ().

Is this correct?

Kind regards
André

On Wednesday, 3 August 2016 at 17:16:10 UTC, Andre Pany wrote:
> Hi,
>
> I just stumbled over this behavior. I am not sure whether
> the behavior is correct or not.
> [...]

> alias foo = () => new Object;
...is an alias for a delegate/function returning an Object. It is analogous to
> alias foo = () { return new Object; };

> void bar(Object o){}
...is a function accepting an Object parameter. In main you are trying to call this overload
> void bar(Object function() o){}
...which is essentially
> void bar(typeof(foo) o) {}

You can use pragma(msg, typeof(symbol).stringof) to inspect what types are being thrown around.
> void bar(T)(T t)
> {
>     pragma(msg, T.stringof); // "Object function() pure nothrow @safe"
> }
>
> void main()
> {
>     auto n1 = foo;
>     pragma(msg, typeof(foo).stringof); // "Object function() pure nothrow @safe"
>     bar(foo);
> }

You could argue that parameterless function call rules should apply here and have it implicitly converted to bar(foo()), but it doesn't. I imagine the ambiguity (of delegate vs function return value) would just cause more problems than it would solve.

On Wednesday, 3 August 2016 at 18:15:23 UTC, Anonymouse wrote:
> On Wednesday, 3 August 2016 at 17:16:10 UTC, Andre Pany wrote:
>> [...]
>
>> [...]
> ...is an alias for a delegate/function returning an Object. It is analogous to
>> [...]
>
>> [...]
> ...is a function accepting an Object parameter. In main you are trying to call this overload
>> [...]
> ...which is essentially
>> [...]
>
> You can use pragma(msg, typeof(symbol).stringof) to inspect what types are being thrown around.
>> [...]
>
> You could argue that parameterless function call rules should apply here and have it implicitly converted to bar(foo()), but it doesn't. I imagine the ambiguity (of delegate vs function return value) would just cause more problems than it would solve.

Thanks for the info. Yes, I forgot the () for new Object;
Adding the () for new Object() still returns the same error.
I think you are right with the ambiguity.

Kind regards
André

Function pointers and delegates are not intended to allow optional parentheses. See also DIP23.

On 08/03/2016 09:40 PM, Andre Pany wrote:
> Thanks for the info. Yes, I forgot the () for new Object;
> Adding the () for new Object() still returns the same error.

`new Object` without parentheses is perfectly fine.